Plasma Assisted Catalytic Conversion of CO₂ and H₂O Over Ni/Al₂O₃ in a DBD Reactor

Abstract

We present an innovative approach for reacting carbon dioxide and water to give syngas by combining heterogeneous catalysis and non-thermal plasma techniques. This approach utilizes an abundant water and nickel catalyst, and mitigates the thermodynamic penalty by using a Dielectric Barrier Discharge (DBD) plasma reactor. Argon dilution was used in the experiment to reduce the exothermic recombination of hydrogen and oxygen, which is considered as the major hurdle for H₂O conversion. As a result, the syngas ratio was dramatically improved from 0.07 to 0.86. In addition, the conversions of CO₂ and H₂O were improved by packing Ni/γ–Al₂O₃ catalysts into the DBD reactor. The yields of H₂ and CO were up to 13.8% and 5.6% respectively. The conditions for plasma catalysis and the catalyst characterization are presented and discussed.

Introduction

Global warming is a serious environmental issue that is exacerbated by the greenhouse effect. Mankind can work towards reducing this effect by decreasing the emission of anthropogenic greenhouse gases. Of these, CO₂ from fuel combustion is by far the largest, accounting for 72% of the total emissions [1]. According to the analysis by the National Oceanic and Atmospheric Administration of the USA in 2016, the global annual temperature has increased at an average rate of 0.07 °C per decade since 1880 and at an average rate of 0.17 °C per decade since 1970 [2]. The atmospheric concentration of CO₂ keeps an upward tendency from 370 ppm in 2000 to 405 ppm in 2018 [3]. To control the concentration of CO₂ in the air, CO₂ capture and utilization could be one of the attractive solutions for the mitigation of CO₂ emissions.

Normally, hydrogen is used as co-reactant in the conventional CO₂ to fuel synthesis. H₂O is not only a cheaper hydrogen source compared with H₂ and CH₄, but also the common waste emitted with CO₂ in industrial processes such as ammonia production. The direct conversion of CO₂ and H₂O is a promising approach based on the use of cheap, abundantly available raw materials.

$${\text{CO}}_{{2\left( {\text{g}} \right)}} + {\text{H}}_{2} {\text{O}}_{{\left( {\text{g}} \right)}} = {\text{CO}}_{{\left( {\text{g}} \right)}} + {\text{O}}_{{2\left( {\text{g}} \right)}} + {\text{H}}_{{2\left( {\text{g}} \right)}} \quad \Delta {\text{H}}_{\text{r}}^{\text{o}} = 525\,{\text{kJ/mol}}$$

(1)

$${\text{CO}} + {\text{H}}_{2} {\text{O }} \rightleftharpoons {\text{CO}}_{2} + {\text{H}}_{2} \quad \Delta {\text{H}}_{\text{r}}^{\text{o}} = - 41\,{\text{kJ/mol}}$$

(2)

However, converting CO₂ and H₂O incurs a large thermodynamic penalty, requiring high temperature to break chemical bonds (1) due to the chemical stability of both components. This may be followed by a Water Gas Shift Reaction (WGSR) as shown in (2). Even at 2000 K, CO₂ conversion is only 1.5% [4]. Existing approaches for CO₂ and H₂O conversion include electrolysis and photocatalysis, but they have various shortcomings such as high operation temperature in electrolysis [5] and low efficiency of solar energy utilization in photocatalysis [6]. Here, we turn to non-thermal plasma, which can be generated by electrical discharges at atmospheric pressure. It provides a way to convert CO₂ and H₂O to syngas or hydrocarbons at low temperature and atmospheric pressure. Moreover, non-thermal plasma can be powered by electricity generated from renewable sources such as solar and wind, incorporating the CO₂ molecules into a renewable carbon cycle that can reduce the dependence on fossil fuels. In addition, this method is suitable decentralized and small-scale CO₂ conversion, which can be coupled to various CO₂ sources at suitable locations.

The conversion of CO₂ and H₂O by non-thermal plasma was studied in DBD reactors [7, 8], microwave discharge reactors [9, 10], gliding arc discharge reactors [11], surface discharge reactors [12] and ferroelectric pellets packed-bed reactors [13]. However, the splitting of CO₂ and H₂O molecules remains a challenge. The main product reported in those researches is syngas with different H₂: CO ratios. Methane [8] and dimethyl ether [12] have also been observed, but not quantified. Guo et al. [9] studied the simultaneous dissociation of CO₂ and H₂O in a surface wave sustained discharge operating at 915 MHz in a pulse regime, the highest yields of CO and H₂ are lower than 8% and 4%, respectively, at a flow rate of 6 slm. They added NiO/TiO₂ treated with Ar plasma in the CO₂ and H₂O reaction to explore the synergy effect between plasma and catalysts. The conversion of CO₂ was improved from 23% (with plasma only) to 43%, with syngas as the sole product.

DBD is the discharge generated between two electrodes separated with an insulating barrier, which is widely applied in CO₂ conversion by non-thermal plasma [14]. Catalysts can be packed into the discharge zone of the DBD reactor and work with plasma synergistically. Ramses et al. [7] studied the conversion of CO₂ and H₂O with H₂O concentration in the feed gas from 0 to 8% in a DBD reactor. The conversions of CO₂ and H₂O were < 5% and syngas was produced with various H₂/CO ratios up to 0.18. The ratio increased linearly with the increase of water content. Shaik et al. [8] reported that the CO₂ conversion ascends from 24% with plasma to 36% when combining the plasma with Ni/Al₂O₃ catalysts. Carbon nanofibers were obtained in the packing region of the DBD reactor and CH₄ was detected besides syngas in the product stream. However, the water conversion and the amount of methane produced were not mentioned in detail.

Our main aim here is to evaluate whether the addition of Ar and catalysts can increase the syngas ratios (H₂:CO) and the conversions of CO₂ and H₂O. First, CO₂ and H₂O with various feed gas compositions were converted to syngas directly in the DBD reactor. Then, to improve the syngas ratio and H₂O conversion, different contents of Ar were added in the feed gases for the purpose of avoiding the recombination of H₂ and O₂. Finally, Ni/γ–Al₂O₃ catalysts were combined with plasma to improve the syngas ratios of the product stream and obtaining CH₄.

Experimental

Experimental Setup

The experiments are carried out in a cylindrical DBD reactor as shown in Fig. 1. A stainless-steel mesh (ground electrode) is wrapped over the outside of a quartz tube with outer and inner diameters of 13 mm and 10 mm, respectively (dielectric thickness = 1.5 mm). A stainless-steel rod with an outer diameter of 8 mm is placed in the center of the quartz tube and it acts as an inner electrode. The length of the discharge region is 100 mm with a discharge gap of 1 mm, which resulted in a discharge volume of 2.83 cm³. The inner electrode is connected to a high voltage output and the outer electrode is grounded via an external capacitor (100 nF). The DBD reactor is supplied by an AC high voltage–power supply (AFS generator G15S-150 K) for a maximum power of 1500 W, with a maximum peak-to-peak voltage of 60 kV and a frequency range of 10–150 kHz. The applied voltage (V₁) is measured by a high voltage probe, while the voltage (V₂) on the external capacitor is measured by another voltage probe. All of the electrical signals are measured by a four-channel digital oscilloscope. The Q–U Lissajous method is used to calculate the discharge power. The specific energy input (SEI) is equal to the ratio of the calculated discharge power to the gas flow, as in (3). The output products are detected by Gas Chromatography (Thermal Scientific Trace 1300) directly with Flame Ionization Detector (FID) for CH₄ and Thermal Conductivity Detector (TCD) for other gases.

Fig. 1

Schematic diagram of the experimental setup

$${\text{SEI }}\left[ {{\text{J cm}}^{ - 3} } \right] = \frac{{{\text{discharge power }}\left[ {\text{W}} \right]}}{{{\text{flow rate }}\left[ {{\text{ml min}}^{ - 1} } \right]}} \cdot \frac{{60\left[ {{\text{s min}}^{ - 1} } \right]}}{{1\left[ {{\text{cm}}^{3} {\text{ml}}^{ - 1} } \right]}}$$

(3)

In the plasma catalytic section, the catalysts were fully packed into the discharge volume within the 1 mm gap. The reactions with CO₂ and H₂O were carried out using plasma alone, plasma with support (Al₂O₃, TiO₂) and plasma with catalysts (Ni/Al₂O₃) in the DBD reactor. The feeding gases, CO₂ and Ar were provided from gas cylinders and controlled by a set of mass flow controllers. H₂O was evaporated and mixed with CO₂ in a controlled evaporator mixer. The gas line and the reactor were heated to 105 °C in order to avoid water condensation during reaction. The gas flow rates of CO₂, H₂O and Ar can be adjusted at ranges of 0–200 mL/min, 0–20 ml/min and 0–200 ml/min, respectively. The conversion, yield of CO₂ and H₂O were calculated by comparing the CO₂ and H₂O peak areas of the GC before and after plasma, processing based on following equations,

$${\text{CO}}_{2} {\text{conversion }}\left[ \% \right] = \frac{{{\text{moles of CO}}_{2} {\text{converted}}}}{{{\text{moles of CO}}_{2} {\text{in feed}}}} \cdot 100{\text{\% }}$$

(4)

$${\text{Yield of CO}}\left( {{\text{H}}_{2} } \right)\left[ \% \right] = \frac{{{\text{moles of CO}}\left( {{\text{H}}_{2} } \right) {\text{produced}}}}{{{\text{moles of CO}}_{2} \left( {{\text{H}}_{2} {\text{O}}} \right) {\text{in feed}}}} \cdot 100{\text{\% }}$$

(5)

To analyze the products, two different selectivities are defined, C-based selectivity for the C-containing species (e.g. CO, CH₄) and the H-based selectivity for the H-containing species (e.g. CH₄, H₂).

H-based selectivity as Eqs. (6, 7):

$${\text{S}}_{{{\text{H}},{\text{H}}_{2} }} = \frac{{{\text{moles of H}}_{2} {\text{produced}}}}{{{\text{moles of H}}_{2} {\text{O consumed}}}}$$

(6)

$${\text{S}}_{{{\text{H}},{\text{CH}}_{4} }} = \frac{{{\text{moles of CH}}_{4} {\text{produced }}}}{{{\text{moles of H}}_{2} {\text{O consumed}}}} \cdot 2$$

(7)

C-based selectivity as Eqs. (8, 9):

$${\text{S}}_{{{\text{C}},{\text{CO}}}} = \frac{\text{moles of CO produced}}{{{\text{moles of CO}}_{2} {\text{consumed}}}}$$

(8)

$${\text{S}}_{{{\text{C}},{\text{CH}}_{4} }} = \frac{{{\text{moles of CH}}_{4} {\text{produced }}}}{{{\text{moles of CO}}_{2} {\text{consumed}}}}$$

(9)

Catalyst Preparation

Ni/γ–Al₂O₃ catalysts were prepared by wet impregnation, using nickel nitrate hexahydrate Ni(NO₃)₂·6H₂O, 98.5%, Sigma-Aldrich) as a Ni precursor and commercial Al₂O₃ (Sigma-Aldrich) as support. The right amount of precursor was used to obtain 10, 20 and 30 wt% of metal loading. The formed slurry was stirred for 24 h at 60 °C, followed by 2-h drying at 120 °C and calcination in an open-air furnace at 450 °C for 4 h to obtain NiO/γ–Al₂O₃. Before the experiments, the catalysts were reduced in 20% H₂/Ar flow with a flow rate of 30 ml/min at 700 °C for 1 h and are denoted as Ni/γ–Al₂O₃.

Characterization of the Catalysts

The following techniques were used to characterize the catalysts before and after the CO₂ and H₂O reactions in the plasma-catalytic reactor. The surface area of the catalysts was determined by N₂ adsorption at a − 196 °C, using BET analysis. Temperature programmed reduction (TPR) was applied to check the reducibility of the catalysts. Before TPR measurements, the catalyst samples were heated in He at 300 °C for 1 h and flushed with Ar for 1 h at 50 °C. Afterwards, the sample was cooled down in flowing Ar to room temperature. The Ar flow was changed to 10% H₂/Ar and the reactor was heated linearly at a rate of 10 °C/min up to 900 °C. The H₂ consumption was detected by a thermal conductivity detector (TCD). Analysis of the crystalline structure of the catalyst was conducted by X-ray Diffraction (XRD) at a scanning rate of 0.02°/20 s from 2θ 10°–90°. The mass loss of the catalysts was measured by Thermogravimetric Analysis (TGA) in air by heating up to 1000 °C at a rate of 10 °C/min. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used to observe the particle size of the catalysts and the structure of the samples.

Results and Discussion

Effect of H₂O Content on the Conversion of CO₂ and H₂O

The impact of the H₂O content from 0 to 10% on the conversion of CO₂ and H₂O was examined at a total rate of 100 ml/min in a blank DBD reactor as shown in Fig. 2. The operated voltage and frequency were 14 kV (from peak to peak) and 30 kHz respectively. The value of SEI was 55.4 J/cm³ for all the reactions, which was calculated by integrating the area of Q–V Lissajous figure. As the concentration of H₂O increased, the conversion of CO₂ decreased from 8.7 to 2.9% and the conversion of H₂O decreased slightly from 1.7 to 1.0%, as shown in Fig. 2.

Fig. 2

CO₂ and H₂O conversion as a function of the water vapor content

The product compositions and syngas ratios are shown in Table 1. The main product is syngas with different H₂/CO ratios. The addition of H₂O led to a decline of the CO₂ conversion and an increase of H₂ production, resulting in a rise of syngas ratio in product, this is in line with reported literature [7, 11, 13]. In this way, the syngas ratio can be adjusted by changing the H₂O/CO₂ ratio in the feed gases to fit the processing needs of gas fermentation. However, the current H₂O conversion is less than 2% and the syngas ratio is too low to be applied in the industrial production.

Table 1 Product composition after plasma treatment

There are two possible reasons for the low H₂O conversion are as follows: (1) The addition of H₂O vapor in the feed gas leads to a reduction in the number of micro-discharges [15] and electron density [7], resulting in low conversions of CO₂ and H₂O [11]. (2) Rapid recombination reactions in plasma limit both of CO₂ and H₂O feed gases conversions and syngas ratios in the product.

Rate coefficient k (cm³ molecule⁻¹ s⁻¹) [7]

$${\text{CO}}_{2} + {\text{e}}^{ - } \to {\text{CO}}_{{}} + {\text{O}}_{{}} + {\text{e}}^{ - } \quad { - }$$

(10)

$${\text{H}}_{2} {\text{O}} + {\text{e}}^{ - } \to {\text{OH}} + {\text{H}} + {\text{e}}^{ - } \quad { - }$$

(11)

$${\text{CO}} + {\text{OH}} \to {\text{CO}}_{2} + {\text{H}}\quad 2.8 \times 10^{ - 14}$$

(12)

$${\text{H}} + {\text{OH}} \to {\text{O}} + {\text{H}}_{2} \quad 2.1 \times 10^{ - 16}$$

(13)

$${\text{H}}_{2} + {\text{OH}} \to {\text{H}}_{2} {\text{O}} + {\text{H }}\quad 1.1 \times 10^{ - 14}$$

(14)

$${\text{O}} + {\text{O}} + {\text{M}} \to {\text{O}}_{2} + {\text{M}}\quad 7.0 \times 10^{ - 33}$$

(15)

$${\text{H}} + {\text{O}}_{2} + {\text{M}} \to {\text{HO}}_{2} + {\text{M }}\quad 1.4 \times 10^{ - 32} - 2.9 \times 10^{ - 31}$$

(16)

$${\text{HO}}_{2} + {\text{O}} \to {\text{O}}_{2} + {\text{OH}}\quad 5.4 \times 10^{ - 11}$$

(17)

$${\text{H}} + {\text{OH}} + {\text{M}} \to {\text{H}}_{2} {\text{O}} + {\text{M}}\quad 1.5 \times 10^{ - 30} - 2.2 \times 10^{ - 29}$$

(18)

The possible reaction path was listed as (10–18), the rate coefficient of related reactions was reported in a study of zero-dimension kinetic model by Ramses [7]. The formation of CO and H₂ are mainly through electron impact induced reaction (10) and (11). CO obtained from CO₂ dissociation can recombine with OH to form CO₂ rapidly and gives rise to the reduction of CO₂ conversion (12). This equation shows a higher reaction rate than the following reactions like H₂ production (13) and three body recombination of O₂/H₂O (15, 18), which was considered as the main reason leading to the low conversion of CO₂ [7]. The H atoms produced in Eqs. (11, 12) react further to generate H₂, OH, H₂O as Eqs. (13, 17) and react with OH back to H₂O through three body recombination process (18), resulting in the low H₂O conversion. Other species such as O₃ and H₂O₂ can also form in theory, but analysing these is out of the scope of this paper.

Effect of Ar on the Conversion of CO₂ and H₂O

The conversion of CO₂ in DBD reactor rises dramatically with the addition of Ar [16]. The breakdown voltage is lower in the CO₂/Ar mixtures owing to the higher Townsend ionization coefficient of Ar, this indicates that a larger fraction of the power can be utilized effectively for the CO₂ dissociation, as less power will be dissipated for the gas breakdown [16]. The charge could be transferred from the Ar⁺ ions to CO₂, which promotes the CO₂ splitting as shown in (19, 20). However, the ionisation of Ar requires a much higher electron energy (15.76 eV) than that for the excitation of Ar such as 11.55 eV for Ar (4s³P²), and 11.72 eV for Ar (4 s′³P⁰) [17]. Thus, Ar is more likely excited to its metastable state rather than being ionized as given in the (21). The presence of the metastable Ar species (Ar*) could create new reaction pathways for the dissociation of CO₂ as in (22) [18]. As H₂O conversion was lower than 2% in the tests (Sect. 3.1), the effect of Ar was explored in the reaction to dilute the species generated in the reaction and avoid the recombination reactions.

$${\text{Ar}}^{ + } + {\text{CO}}_{2} \to {\text{Ar}} + {\text{CO}}_{2}^{ + }$$

(19)

$${\text{CO}}_{2}^{ + } + {\text{e}}^{ - } \to {\text{CO}} + {\text{O}}$$

(20)

$${\text{e}}^{ - } + {\text{Ar}} \to {\text{e}}^{ - } + {\text{Ar}}^{*}$$

(21)

$${\text{Ar}}^{*} + {\text{CO}}_{2} \to {\text{Ar}} + {\text{CO}} + {\text{O}}$$

(22)

The impact of the addition of 10%, 50% and 90% of Ar in the feed gases on the conversion of CO₂ and H₂O was studied at a total rate of 100 ml/min in the DBD reactor as shown in Fig. 3. We could only test the H₂O content up to 5% in 90% Ar, due to the limitation of the setup. The operated voltage and frequency were 14 kV (peak to peak) and 30 kHz, respectively. Generally, as the concentration of Ar increases, the conversion of H₂O has an increasing tendency from 1 to 4.1%. However, the conversion of CO₂ decreased from 8.7 to 3.1%. As for the tendency described in Sect. 3.1, the conversions of both CO₂ and H₂O showed a decreasing trend with the rise of H₂O content. The products are syngas with various ratios. The values of SEI and syngas ratio are shown in Table 2.

Fig. 3

The effect of Ar on the conversion of CO₂ (a) and H₂O (b) at different H₂O contents

Table 2 Specific energy input (SEI) values and syngas ratios in the product stream

Adding Ar promoted the conversion of H₂O, while the conversion of CO₂ decreased simultaneously. The tendency is different from the effect of Ar on the conversion of CO₂ alone [16]. The syngas ratios follow an upward tendency up to 0.86 with the increase of the H₂O content. The dissociation energy of H₂O (2.6 eV/mol) is slightly lower than that for the CO₂ dissociation ($2.9{\text{eV}}/{\text{mol}})$. Charge transfer between the reactive Ar species (e.g. Ar^*, Ar⁺ ions) and H₂O might contribute to the H₂O dissociation. As a result, it promotes reverse reaction of CO₂ leading to the decrease of the CO₂ conversion. When the H₂O content is increased, the conversions of both CO₂ and H₂O are decreased in all cases. The reason could be that the addition of H₂O leads to a reduction in electron density as discussed in Sect. 3.1.

Catalytic Effect on the Conversion of CO₂ and H₂O in the DBD Reactor

As summarized in the above discussion, both H₂O conversion and syngas ratio were enhanced with the addition of Ar. However, the conversion of CO₂ and H₂O are still lower than 10%; besides, syngas ratio is slightly lower than the one for acetic acid production by Fischer–Tropsch synthesis. To improve the conversion and obtain hydrocarbons directly from the reaction, catalysts were packed into DBD reactor. Supported Ni catalysts were proved to enhance the conversions in CO₂–CH₄ dry reforming reaction by non-thermal plasma [19,20,21,22]. It also was used to improve the selectivity of CH₄ for photocatalytic CO₂ reduction by H₂O [23]. Thus, supported Ni catalysts were chosen as the catalysts in our experiments. The reaction with 90% Ar showed highest conversion of H₂O. Lower H₂O content supports the CO₂ conversion, thus, the following experiments were performed with 90% Ar, 2% H₂O and 8% CO₂.

Flow Rate Tests for Reaction

To study the effect of the flow rate on the reaction, 70 ml/min (lowest flow rate of the setup), 100 ml/min, 150 ml/min and 200 ml/min were applied in the plasma reaction. The operated voltage and frequency were 14 kV (from peak to peak) and 30 kHz, respectively. The conversions of CO₂ and H₂O are shown in Fig. 4.

Fig. 4

Conversion of CO₂ and H₂O with various flow rates

The conversions of feed gases showed a downward trend with a rise of total flow rate. The highest conversion of CO₂ and H₂O was obtained at the lowest flow rate of 70 ml/min of feed gas. It coincides with the results in the CO₂ dissociation [24] and CO₂ hydrogenation in DBD reactor [25]. Low residence time leads to the low conversion of feed gases. Therefore, the optimal/lowest flow rate of 70 ml/min was selected for plasma catalytic experiments.

Plasma Catalytic Reactions: Effect of γ-Al₂O₃ and TiO₂

γ-Al₂O₃ and TiO₂ are the most common catalyst supports in plasma-catalytic reactions, which are chosen to compare the effects on the reaction resulting from various dielectric constants and specific surface areas. The first plasma-catalytic experiments were performed with these two supports. Experiment with blank reactor was also performed for comparison. The conversion of CO₂ and H₂O as a function of time is presented in Fig. 5, and the Q–V Lissajous figures of these three cases are presented in Fig. 6. The operated voltage and frequency were 12 kV from peak to peak (the highest stable voltage for a long-term run) and 30 kHz in the reaction, respectively. Because Al₂O₃ has a strong water absorption ability, product measurements were taken after 15 min of plasma treatment. In this way, the adsorbed water is released before the measurement so that the measurement results will not be affected. The detected products were CO and H₂. Figure 5 shows that TiO₂ was effectively enhanced CO₂ conversion, around 20–24% CO₂ conversion was achieved, which is 15% higher than the case when no catalyst is used. However, TiO₂ had no obvious effect on H₂O conversion. The reaction with γ-Al₂O₃ promotes CO₂ conversion from 5% (only plasma) to 10% and H₂O conversion from 1.6 to 2.5%.

Fig. 5

Conversion of CO₂ (a) and H₂O (b) using different supports

Fig. 6

Lissajous figures of reaction in DBD reactor without and with different packing material

The shape of the Lissajous figure changes from parallelogram to oval shape when either TiO₂ or Al₂O₃ are fully packed in the DBD reactor, resulting in the change of discharge mode from typical filamentary discharge to a combination of weak filamentary and predominantly surface discharge. The pellets fully packed into the discharge gap result in significant reduction in the discharge volume, limiting the distance where filamentary microdischarges can form [26]. As a result, less intense filament current peaks can be generated in the void between pellet–pellet and pellet–quartz wall as shown in Fig. S1. Meanwhile, a surface discharge can be formed on the surface of pellets near the contact points between the pellets, which is introduced as a packed-bed discharge effect [26, 27]. The breakdown voltage of the discharge does not change significantly with/without the packing materials from Fig. 6. The different discharge behaviors with packing pellets are caused by the enhanced local electric field strength near the contact points between the pellets and the pellet–dielectric wall [28]. From the measured V–I characteristics shown in Figure S1, differences in the number of microdischarges are observed with an order as: Blank > Al₂O₃ > TiO₂. In addition, the SEI in the case of TiO₂ is slightly higher than Al₂O₃ (29.3 J/cm³ and 25.8 J/cm³ respectively), which could also contribute to the difference in conversion of CO₂. The difference in discharge characteristics between Al₂O₃ and TiO₂ is not significant. We believe that the main reason for the difference in conversion is the physical properties of the packing material. The surface area of TiO₂ is much smaller than Al₂O₃ (Table S3). Indeed the plasma generation in micropores is less likely to occur, however, Al₂O₃ with large surface area and strong H₂O absorption capacity can absorb more H₂O on the surface compared with TiO₂ at the same flow rate, leading to higher residence time of H₂O on the surface of pellets which promotes the H₂ production. At the same time more OH is produced, which will react with CO, resulting in a lower conversion of CO₂. On the other hand, the rate of CO reverse reactions is mainly determined by the surface rather the gas phase. Therefore, TiO₂ with lower surface area gives a lower rate for CO to react with oxygen species, hence higher CO₂ conversion is observed [29].

Since γ-Al₂O₃ showed the best H₂O conversion results, it was used as the support for the following experiments. The introduction of Ni/γ–Al₂O₃ catalysts in the plasma discharge was reported to improve the performance of CO₂ activation reactions with hydrocarbons or with H₂O, in comparison with the plasma-only reaction [8, 30, 31]. Thus, Ni/γ–Al₂O₃ was used to explore the catalytic effect on this reaction further.

Plasma Catalytic Reactions: Effect of Ni/γ–Al₂O₃ on the Conversion of CO₂ and H₂O

To study the effect of different metal loadings on the catalytic activity, 10% Ni/Al₂O₃, 20% Ni/Al₂O₃ and 30% Ni/Al₂O₃ catalysts were studied and compared with blank experiments. The results are presented in Fig. 7.

Fig. 7

Conversion of CO₂ (a) and H₂O (b) with no catalyst, Al₂O₃, 10% Ni/Al₂O₃, 20% Ni/Al₂O₃, 30% Ni/Al₂O₃ and 20% NiO/Al₂O₃

From Fig. 7, 10% Ni/Al₂O₃ significantly showed a positive effect on the conversion of CO₂, converting between 10 and 14% of CO₂ and showing the best CO₂ conversion results compared with the others in 2-h reaction. It also has slightly higher conversion of H₂O compared with the blank and Al₂O₃. CO₂ conversion with 30% Ni/Al₂O₃ catalyst was similar to Al₂O₃, while H₂O conversion was slightly higher. It is evident that 20% Ni/Al₂O₃ shows the lowest CO₂ conversion values compared with other conditions. However, it is the most active for H₂O conversion. Although the conversion declines from 14.7 to 4% after 120 min, these values are always higher than the H₂O conversions in the other cases.

Furthermore, the products in all cases were syngas with different ratios, apart from 292 ppm CH₄ obtained with 20% Ni/Al₂O₃. H₂ and CO yields were up to 13.8% and 5.6%, respectively (Fig. 8). CH₄ achieved the highest yield 0.32% at 15 min of reaction (first measurement) and it was not produced anymore after 75 min of reaction. The highest syngas ratio was 0.6 after 15 min due to the high yield of H₂, and it decreased to 0.2 after 2 h of reaction. Thus, metallic Ni is more active than NiO for H₂O conversion, and it can produce CH₄ from Figs. 8 and 9.

Fig. 8

Yield of valuable products in the reaction using 20% Ni/Al₂O₃ catalyst

Fig. 9

Selectivity of valuable products in the reaction with 20% Ni/Al₂O₃

The C-based selectivity and H-based selectivity are depicted in Fig. 9. The C-based selectivity of CH₄ decreased from 7 to 0% with time. Reversely, the selectivity of CO increased from 93 to 100% after 75 min of reaction. Besides, the maximum value of H-based CH₄ selectivity was 11%, descending with reaction time.

The ability of Ni/Al₂O₃ to increase CO₂ conversion was observed previously [8], which is in accordance with results of 10% Ni/Al₂O₃. However, Ni/Al₂O₃ catalysts are also active for the Water Gas Shift (WGS) reaction (2) [32], which could explain the catalytic behavior of 20% Ni/Al₂O₃ catalyst. The CO produced from CO₂ dissociation could react with H₂O, promoting the production of H₂ and CO₂ by the WGS reaction. However, the conversion decreased with time in 90 min. One possible explanation for this behavior is that CO dissociated to C (Boudouard reaction), reducing the conversion of H₂O. This carbon can react with H₂ further to produce CH₄, as shown in Fig. 10 [18]. When the H₂ concentration in the product mixture decreased, CH₄ production decreased to zero in an hour. It was also reported that CO₂ by in situ formation of hydrogen during water splitting with Ni catalysts may result in the production of CH₄ and PCVD of CH₄, leading to the formation of carbon fibers in a DBD reactor [8]. Because CH₄ was detected during the 20% Ni/Al₂O₃ reaction, so the other possible reason is that the formation of carbon filaments from the PCVD of CH₄ may reduce the catalyst activity and H₂ production.

Fig. 10

Reprinted with permission from [18], copyright 2017 Springer

Possible major reactions for the formation of CH₄ on the Ni/Al₂O₃ catalyst.

To conclude, we have seen that inserting packing materials in the DBD plasma discharge can enhance the conversions compared with plasma alone. 10% Ni/Al₂O₃ shows the best CO₂ conversion (14%) but less significant for H₂O conversion. On the other hand, the reaction with 20% Ni/Al₂O₃ catalyst present much higher H₂O conversion (14.7%) and lower CO₂ conversion (5.9%) compared with 10% Ni/Al₂O₃, suggesting 20% Ni/Al₂O₃ is more effective for side reactions (WGS) and promotes H₂ production. Further, the CO₂ conversion of 20% Ni/Al₂O₃ reaction is lower than that in blank experiments, indicating that the WGS reaction occurs during 20% Ni/Al₂O₃ reactions and reduces CO production. Unlike 20% Ni/Al₂O₃, all the CO₂ and H₂O conversions in other cases were enhanced compared with blank experiments, suggesting that the dissociation reactions of CO₂ and H₂O to CO and H₂ play a dominating role. Besides, it is also possible that Ni reacts with H₂O to produce NiO and H₂ (Ni + H₂O = NiO + H₂) during the reaction and increases the H₂ production at the beginning of reaction with 20% Ni. The experiments with 30% Ni gave low conversions, suggesting the thermal catalysis and sintering may occur especially in the case of 30% Ni/Al₂O₃. Further characterization of the materials was performed to study the observed catalytic behavior.

Catalysts Characterization

X-ray Diffraction (XRD) Measurements

The XRD patterns for the 10% Ni/Al₂O₃, 20% Ni/Al₂O₃, 30% Ni/Al₂O₃ catalysts after reduction and after reaction are shown in Fig. S6. The characteristic peaks of NiO are not obvious in 10%/20% Ni and absent from the 30%Ni sample. Therefore, it cannot be confirmed that NiO formation is the dominating factor. The Ni peak of 30% Ni/Al₂O₃ after reaction is more intense than the peak before reaction unlike peaks of 10%/20% Ni/Al₂O₃. The average crystallite size of Ni in 30% Ni/Al₂O₃ catalyst was estimated by the Scherrer formula using the half width of the Ni (200) peak, which was 16.5 nm after reduction and 25.3 nm after reaction. This shows that sintering of Ni particles takes place during the reaction with 30% Ni/Al₂O₃.

Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) Analysis

TEM and SEM analysis of the catalysts after reduction (noted as before reaction) and after reaction were performed in order to study the formation of carbon deposits. Compared with the clean surface of reduced Ni/Al₂O₃ samples (before reaction) shown in Fig. S7. (a, c, e) from SEM images, filamentary deposits were clearly present on the surface of the catalysts after their use in the reaction for 2 h [Fig. S7. (b, d, f)]. This carbon can be identified as filamentary carbon [33, 34]. With TEM images, the amount of carbon filaments in 10% Ni/Al₂O₃ catalyst is small, making it difficult to identify. From Fig. S7 (a, b), the carbon filaments are not encapsulating the particles, therefore they do not influence the catalytic activity of this material. For 20% Ni/Al₂O₃, more carbon filaments are observed [Fig. S8 (c, d)]. They stay closer to the metal clusters, causing partial encapsulation. Fig. S8 (e, f) clearly shows that the carbon formed on the surface of 30% Ni/Al₂O₃ catalyst is encapsulating the metal particles. Curiously, those filaments form a V shape. Deactivation of the Ni catalysts by coking is a common problem [35, 36]. Coking can be produced from two different reactions: the Boudouard reaction (2CO → CO₂ + C) and/or the hydrocarbon decomposition. It is possible that CH₄ is produced, but it decomposes to C and H₂.

In summary, the amount of metal loading strongly affects the properties of the final catalysts. 30% Ni/Al₂O₃ catalyst has bigger metal clusters with a weak interaction with the support (Fig. S5) and a significantly lower surface area than the other two materials (Table. S3), which may be the main reason for its poor catalytic activity. Besides, temperature gradients at the catalyst surface in DBD reactor may arise and influence the overall catalyst activity [37]. The heat capacity of metals Ni (C_p = 0.44 J g⁻¹ K⁻¹) is typically lower than the heat capacity of most commonly dielectric support materials Al₂O₃ (C_p = 0.72 J g⁻¹ K⁻¹) while the thermal conductivity of metals is much higher (κ = 90.9 W m⁻¹ K⁻¹ for Ni, κ = 20 W m⁻¹ K⁻¹ for Al₂O₃) [37]. Therefore, the temperature will be higher in the Ni surface in the plasma reaction leading to the sintering and deactivation of catalysts especially for 30% Ni/Al₂O₃. Moreover, it induces the formation of encapsulating carbon filaments that block the active sites. For 20% Ni/Al₂O₃ catalyst, it is the least active material for CO₂ conversion, but the best catalyst for H₂O conversion. This behavior may reflect the presence of side reactions such as WGS reaction, methane formation/PCVD and Ni oxidation as explained in Sect. 3.3.3. But Ni oxidation to NiO was not obvious from XRD, suggesting that reducing species such as hydrogen and CO produced in the plasma may react with NiO and reduce it. Among the studied materials, 10% metal loading appeared to be the best one for CO₂ conversion, although it could not convert H₂O. Characterization results (Fig. S5) showed that 10% Ni/Al₂O₃ catalyst contains highly dispersed NiO nanoparticles with a strong interaction with the support, giving a material with a high surface area. In this case the metal-support interface can adsorb more CO₂, while the formation of carbon filaments is partially avoided.

Conclusions

The conversion of CO₂ and H₂O was carried out in a DBD plasma reactor at 105 °C. The effects of argon addition, packed support material and supported catalysts were studied. The addition of Ar produces reactive Ar species and reduces the recombination of H₂ and O₂, which promotes the H₂O conversion. In this case, the syngas ratio can be adjusted to 0–0.86, which approaches the ratio for hydrocarbon synthesis via Fischer–Tropsch synthesis or gas fermentation. Further, the plasma catalytic CO₂ and H₂O reaction was explored with Ni/Al₂O₃ catalyst to improve the conversion of feed gases and to give access to hydrocarbons. Thus, the combination of plasma and a 10% Ni/Al₂O₃ catalyst facilitates the CO₂ conversion. The 20% Ni/Al₂O₃ catalyst promotes the water–gas shift reaction (WGSR) and enhances the H₂O conversion up to 14.7%. In addition, 292 ppm CH₄ was obtained by the plasma reaction, which assumed to be formed from CO through dissociation and hydrogenation. The effect of the 30% Ni/Al₂O₃ catalyst is similar to Al₂O₃ as packing material which is accounted to sintering of the catalyst and carbon deposition.