1 Introduction

The catalytic conversion of CO2 is desirable strategy to not only reduce the CO2 emission but also to produce useful chemicals/fuels [1, 2]. Depending upon the catalysts used, different kinds of products were obtained such as CO via reverse water gas shift (RWGS) reaction, methane (Sabatier reaction) and methanol [3,4,5]. The obtained CO in the RWGS reaction can be converted into value added chemicals through Fischer–Tropsch synthesis. RWGS is endothermic (CO2 + H2 ↔ CO + H2O, ΔHRWGS = + 41 kJ/mol) and thermodynamically favoured at high temperatures [6]. Cu [7], Pt [8] and Rh [9] on various supports have been reported as the most active catalysts for RWGS reaction. Methanation is exothermic (CO2 + 4H2 → CH4 + 2H2O, ΔHSab = − 165 kJ/mol) and thermodynamically favoured at low temperatures [10]. Ni [11], Ru [4] and Rh [12] are most widely used catalysts for CO2 methanation reaction. Cu [13] and Pd [14] are most widely used catalysts for the reduction of CO2 to methanol [15,16,17]. Nickel based catalysts have been widely investigated as catalyst in CO2 hydrogenation reactions owing to its superior catalytic activity and low cost [18, 19]. Recently, nickel based spinel catalysts have been widely used in CO2 hydrogenation reaction due to their low cost and superior catalytic activity [20,21,22]. Further, they were also used in other fields such as in adsorption [23], sensors [24] and as flexible materials [25]. They have also been used as catalyst support due to its low reactivity with the active phase and its high resistance to high temperatures and acidic or basic atmospheres [26]. Interestingly, NiAl2O4 was found to minimize the coke formation in CO2 reforming of methane [27]. Besides nickel based spinels, zinc based spinels were also used in various fields such as in catalysis [15, 28,29,30], adsorption [31] and optics [32] due to their superior catalytic activity and high thermal stability [33]. However, the catalytic applications of these spinel materials for CO2 hydrogenation is not reported. In the present study, various Nickel–Zinc–Aluminum-based spinels as well as oxide/spinel catalysts were produced where the position of the nickel and zinc atoms or ions were changed. The catalysts were characterized by XRD, N2 physisorption, TEM, SEM–EDX and TGA. These catalysts were tested in CO2 hydrogenation reaction in the gas phase. It was found that NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts during the reaction conditions exhibited outstanding activity and selectivity towards methane even at high temperature as these catalysts comprise metallic nanoparticles in their structure. Among these catalysts, NiO/ZnAl2O4 catalyst showed CO2 consumption rate of ~ 19 μmol/g s at 600 °C and ~ 85% as well as ~ 50% of methane selectivity at 450 °C and 600 °C, respectively.

2 Experimental details

2.1 Chemicals

Zn(NO3)2·6H2O (≥ 99%) and Al(NO3)3·9H2O (≥ 98%) were purchased from Sigma-Aldrich. Aqueous ammonia solution was purchased from Molar chemicals. Ni(NO3)2·6H2O was purchased from Merck.

2.2 Catalyst Preparation

The ZnAl2O4 oxide was synthesized by a co-precipitation method in accordance with the procedure reported in the previous work [34]. Typically, appropriate amount of Zn(NO3)2·6H2O and Al(NO3)3·9H2O with a molar ratio of 1:2 were dissolved in 100 mL deionized water. Then, an aqueous ammonia solution was added dropwise into the mixed solution at room temperature until pH value of about 7. The obtained precipitate was aged for 2 h at 70 °C. Then, the solid product was recovered by filtration, washing with deionized water and drying overnight at 100 °C. The ZnAl2O4 was obtained after calcination in air at 500 °C for 5 h. The NiAl2O4 and pure Al2O3 were prepared by the same procedure using their corresponding metal nitrate precursors. In order to investigate the interphase effect of metal cations present in the ZnAl2O4 and NiAl2O4 spinels, we loaded exactly the amount of ZnO present in ZnAl2O4 onto NiAl2O4 and vice versa. Based on the calculation, we loaded 44wt% of ZnO on NiAl2O4 and represented as ZnO/NiAl2O4 and 42wt% of NiO on ZnAl2O4 and represented as NiO/ZnAl2O4.

2.3 Catalyst Characterization

2.3.1 N2 Adsorption–Desorption Isotherm Measurements

The specific surface area (BET method), the pore size distribution and the total pore volume were determined by the BJH method using a Quantachrome NOVA 2200 gas sorption analyzer by N2 gas adsorption/desorption at − 196 °C. Before the measurements, the samples were pre-treated in a vacuum (< ~ 0.1 mbar) at 200 °C for 2 h.

2.3.2 Powder X-ray Diffraction (XRD)

XRD studies of all samples were performed on a Rigaku MiniFlex II instrument with a Ni-filtered CuKα source in the range of 2θ = 10–80°.

2.3.3 Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was obtained using TAQ500 instruments under flow of air from room temperature to 800 °C at a heating rate of 10 °C min−1.

2.3.4 Scanning Electron Microscopy (SEM–EDX)

Scanning electron microscopy equipped with an energy dispersive X-ray spectroscopy (Hitachi S-4700) was applied at 20 kV on the samples.

2.3.5 Transmission Electron Microscopy (TEM)

Imaging of the all the samples were carried out using an FEI TECNAI G2 20 X-Twin high-resolution transmission electron microscope (equipped with electron diffraction) operating at an accelerating voltage of 200 kV. The samples were drop-cast onto carbon film coated copper grids from ethanol suspension.

2.4 Catalytic Activity Studies

2.4.1 Hydrogenation of Carbon-dioxide in a Continuous Flow Reactor

Before the catalytic experiments, the as-received catalysts were oxidized in O2 atmosphere at 300 °C for 30 min and thereafter were reduced in H2 at 300 °C for 60 min. Catalytic reactions were carried out at atmospheric pressure in a fixed-bed continuous-flow reactor (200 mm long with 8 mm i.d.) which was heated externally. The dead volume of the reactor was filled with quartz beads. The operating temperature was controlled by a thermocouple placed inside the oven close to the reactor wall, to assure precise temperature measurement. For catalytic studies, small fragments (about 1 mm) of slightly compressed pellets were used. Typically, the reactor filling contained 150 mg of catalyst. In the reacting gas mixture, the CO2:H2 molar ratio was 1:4, if not denoted otherwise. The CO2:H2 mixture was fed with the help of mass flow controllers (Aalborg), the total flow rate was 50 ml/min. The reacting gas mixture flow entered and left the reactor through an externally heated tube in order to avoid condensation. The analysis of the products and reactants was performed with an Agilent 6890 N gas chromatograph using HP-PLOTQ column. The gases were detected simultaneously by thermal conductivity (TC) and flame ionization (FI) detectors. The CO2 was transformed by a methanizer to methane and it was also analysed by FID. CO2 conversion was calculated on a carbon atom basis, i.e.

$${\text{CO}}_{2 } \,{\text{conversion}}\, (\%) = \frac{{{\text{CO}}_{{2\,{\text{inlet}}}} - {\text{CO}}_{{2 \,{\text{outlet}}}} }}{{{\text{CO}}_{{2 \,{\text{inlet}}}} }} \times 100\%$$

CH4 selectivity and CO selectivity were calculated as following

$${\text{CH}}_{4 } \,{\text{selectivity}}\, (\%) = \frac{{{\text{CH}}_{{4\,{\text{outlet}}}} }}{{{\text{CO}}_{{2\,{\text{inlet}}}} - {\text{CO}}_{{2\,{\text{outlet}}}} }} \times 100\%$$
$${\text{CO}}\,{\text{selectivity}}\, (\%) = \frac{{{\text{CO}}_{\text{outlet}} }}{{{\text{CO}}_{{2\,{\text{inlet}}}} - {\text{CO}}_{{2\,{\text{outlet}}}} }} \times 100\%$$

where \({\text{CO}}_{{2 \,{\text{inlet}}}}\) and \({\text{CO}}_{{2\,{\text{outlet}}}}\) represent the \({\text{CO}}_{2}\) concentration in the feed and effluent, respectively, \({\text{and}}\;{\text{CH}}_{{4\,{\text{outlet}}}} \;{\text{and}}\; {\text{CO}}_{\text{outlet}} \;{\text{represent }}\;{\text{the}}\;{\text{concentration}}\;{\text{of }}\;{\text{CH}}_{4 }\) and CO in the effluent, respectively.

3 Results and Discussion

3.1 X-ray Diffraction (XRD)

The crystal structure of catalysts was investigated by XRD. Figure 1 shows the XRD patterns of NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3. The peaks located at 2θ of 18.9°, 31.38°, 36.67°, 44.39° and 64.88° are assigned to the (111), (220), (311), (400) and (440) planes of the cubic spinel structure of NiAl2O4 respectively (JCPDS Card no. 73-0239) [35]. The peaks located at 2θ of 18.99°, 31.69°, 37.17°, 45.26°, 49.06°, 55.66°, 59.65°, 65.62°, 74.15° and 77.33° are assigned to the (111), (220), (311), (400), (331), (422), (511), (440), (620) and (536) planes of the cubic spinel structure of ZnAl2O4 respectively (JCPDS Card no. 05-0669) [29]. For NiO/ZnAl2O4 and ZnO/NiAl2O4 samples no peaks characteristics of ZnO and NiO are seen indicating fine dispersion of these species on the NiAl2O4 and ZnAl2O4 supports respectively or may be overlapped with the supports diffraction peaks. The peaks located at 2θ of 19.86°, 32.38°, 37.85°, 46.20°, 57.40°, 61.02° and 67.12° are assigned to (111), (220), (311), (400), (422), (511) and (440) planes of the cubic structure of γ-Al2O3 [36].

Fig. 1
figure 1

XRD patterns of NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts

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3.2 N2 Adsorption–Desorption Isotherm

The specific surface area together with the pore volume and pore size was summarized in Table 1. The N2 adsorption–desorption isotherms of ZnO/NiAl2O4 exhibit type IV isotherm with a narrow hysteresis loop of type H3 associated with plate-like particles giving rise to slit-shaped pores [37]. However, Al2O3, NiAl2O4, ZnAl2O4 and NiO/ZnAl2O4 displays type IV isotherms with H2 hysteresis loop at P/P0 = 0.4–1.0 associated with pores with narrow necks and wide bodies, referred to as ‘ink-bottle’ pores [37, 38]. The average pore size distribution is in the range of 2–25 nm indicating the presence of mesopores. After loading ZnO and NiO respectively on NiAl2O4 and ZnAl2O4, the resulting catalyst showed decreased surface area and pore volume.

Table 1 Textural parameters of the catalysts
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3.3 TEM Analysis

The morphology and particle size of the catalysts were examined by TEM measurements and shown in Fig. 2. NiAl2O4 shows spherical shaped morphology with the size of 10 to 20 nm. ZnAl2O4 displays rod like particles. TEM images of the NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts show two separate phases of metal oxides and supports that are well mixed and dispersed which is similar to what have been reported in the literature for NiO/NiAl2O4 catalyst [39].

Fig. 2
figure 2

TEM images of a NiAl2O4, b ZnAl2O4, c NiO/ZnAl2O4 and d ZnO/NiAl2O4

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3.4 SEM–EDX Analysis

Table 2 summarizes the atomic percentages of various elements obtained from the SEM–EDX analyses. SEM–EDX spectra of Al2O3 revealed the presence of Al and O elements with the percentages of 24.21% and 75.79% respectively. All other catalysts also clearly indicates the presence of their corresponding elements.

Table 2 SEM–EDX analysis of the catalysts
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3.5 Catalytic Performances

To explore the catalytic performance, CO2 hydrogenation was performed over the prepared catalysts. Figure 3 depicts the CO2 conversion as a function of temperature over all the catalysts. CO2 conversion and product selectivity are given in Table 3 over all the catalysts. In general, the activity of Ni containing catalysts are remarkably better than that of Zn containing catalysts and Al2O3 catalyst. NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts exhibit highest activity with CO2 conversion of 65% at 600 °C, which is 2.8-fold superior in catalytic activity than that of Al2O3 (Conversion = 23%) and twofold superior in catalytic activity than that of ZnAl2O4 (Conversion = 31%).

Fig. 3
figure 3

CO2 conversion as a function of temperature over NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts

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Table 3 Conversion and selectivity for CO2 hydrogenation over various catalystsa
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Figure 4 depicts the selectivity as a function of temperature for all the studied catalysts. The CO selectivity increases with increasing temperature due to the endothermic RWGS reaction. Among the five systems (NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3) considered in this study, the Ni containing catalysts such as NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 produced CH4 and CO as the product but the Zn containing catalysts such as ZnAl2O4 as well as Al2O3 produced CO as the only product. All the nickel-containing spinels and oxide/spinel structures showed a high selectivity towards methane even at high temperature. NiO/ZnAl2O4 system has a methane selectivity of ~ 85% as well as ~ 50% at 450 °C and 600 °C, respectively.

Fig. 4
figure 4

Selectivity for the CO2 hydrogenation over NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts

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The CO2 conversion exhibit a decrease in the order: NiO/ZnAl2O4 < NiAl2O4 < ZnO/NiAl2O4 < ZnAl2O4 < Al2O3. This can be correlated with increasing Ni content. Given that an increase in Ni content can enhance CO2 hydrogenation activity [40]. The NiO/ZnAl2O4 exhibited 65% CO2 conversion at 600 °C with CH4 and CO as the products. All of the Ni containing catalysts produce CH4 as main products and CO as minor products while ZnO and other Zn containing catalysts as well as Al2O3 produce only CO.

In general, Ni based catalysts produce CH4 through decomposition of formate species to CO and subsequent hydrogenation of adsorbed CO leads to the production of CH4 [41] and ZnO is more active for the RWGS reaction [42]. Table 4 lists the CO2 consumption rates of all the catalysts studied at 600 °C. Figure 5 depicts the CO2 consumption rate as a function of temperature for all the studied catalysts. The CO2 consumption rate is highest on NiO/ZnAl2O4, namely ca. 19.7 μmol h−1 g−1 at 600 °C which was 2.5 times higher than that of Al2O3 (ca. 7.9 μmol h−1 g−1 at 600 °C) catalyst. This catalyst also outperforms other reported spinel catalysts (Table 5) in the CO2 hydrogenation reaction.

Table 4 The CO2 consumption rate (μmol/g.s) at 600 °C in CO2 hydrogenation reaction over NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts
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Fig. 5
figure 5

CO2 consumption rate as a function of temperature over NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts

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Table 5 Comparative table of CO2 consumption rate with the reported spinel catalyst for CO2 hydrogenation
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Although the surface area of Al2O3 was far higher than the NiO/ZnAl2O4, the CO2 consumption rate was far higher on NiO/ZnAl2O4. This was due to presence of metallic Ni under reaction condition in NiO/ZnAl2O4 than in the other catalysts. Comparative table of CO2 consumption rate of the catalyst in this study with the spinel catalyst reported in the literature for CO2 hydrogenation is given in Table 5.

The effect of metal-support interaction was investigated over Ni/SiO2 catalyst in the CO2 hydrogenation reaction [43]. It was reported that the oxygen vacancy present in the support produces surface carbon species and Ni dissociates H2 into atomic hydrogen [44]. In the present study, the high catalytic activity of NiO/ZnAl2O4 catalyst can be attributed to the strong interaction between the Ni and the ZnAl2O4 leading to the incorporation of Ni into the ZnAl2O4 lattice and subsequent formation of oxygen vacancies [45]. This oxygen vacancies produce surface carbon species and the Ni dissociates H2 into atomic hydrogen and forms CO and CH4 as the final products.

3.6 Stability of the Catalyst

Figure 6 shows the stability test of all catalysts for CO2 hydrogenation. For all the catalysts, CO2 consumption rate had no obvious decline with time. This suggested that all the catalysts are more stable during CO2 hydrogenation reaction. The ZnO/NiAl2O4 catalyst showed excellent catalytic stability for CO2 hydrogenation among all the catalysts studied.

Fig. 6
figure 6

Catalytic stability test over NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts at 600 °C

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3.7 Spent Catalysts Characterization

The spent catalysts were characterized by XRD, TGA and TEM.

3.7.1 X-ray Diffraction

The spent catalysts were studied by XRD to elucidate the structural changes. The XRD of spent catalysts after catalytic test are displayed in Fig. 7. All Ni containing spent catalysts show peaks in addition to fresh ones at 2θ = 45.39°, 52.62° and 77° corresponding to the (111), (200) and (220) planes attributed to the metallic nickel (JCPDS No. 04-0850) [51]. However Zn containing spinels and Al2O3 spent catalysts showed almost no changes in their crystalline phases indicating that their crystal structures are more stable during the reaction.

Fig. 7
figure 7

XRD profiles of spent catalysts after catalytic test

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3.7.2 TGA Analysis

TGA was employed to characterize the carbonaceous deposits on the spent catalysts. The TGA and DTG curves of all the spent catalysts were shown in Figs. 8 and 9 respectively. For all the spent catalysts, the weight loss below 200 °C is ascribed to desorption of adsorbed water. This weight loss is also depicted by peak starting at 50 °C and ending at 200 °C in the DTG curve as shown in Fig. 9. For Ni containing catalysts such as NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 both weight loss and weight gain were observed. The weight loss between 200 and 300 °C on NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts were 6.37%, 2.17% and 1.6% respectively. The weight gain above 300 °C on NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts were 2.44%, 4.14% and 2.91% respectively. The weight loss can be attributed to the combustion of amorphous carbon deposit and weight gain can be attributed to oxidation of metallic nickel [52, 53]. XRD also confirms the existence of considerable amount of metallic nickel in the spent catalyst (Fig. 7). As can be seen clearly in the DTG curve, the peak due to weight gain in NiAl2O4 is shifted to higher temperature in comparison to other Ni containing catalysts such as NiO/ZnAl2O4 and ZnO/NiAl2O4 indicates stronger adsorption of carbon deposits on NiAl2O4 than on NiO/ZnAl2O4 and ZnO/NiAl2O4. The weight loss on ZnAl2O4 and Al2O3 catalysts were 4.79% and 10.74% respectively. The weight loss between 200 and 800 °C can be attributed to the burning of carbon deposited over the catalysts [54]. Less carbon was deposited on NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 than on ZnAl2O4 and Al2O3 indicating Ni containing catalysts could effectively reduce carbon deposit. This is in line with their higher catalytic activity in CO2 hydrogenation reaction (Table 4).

Fig. 8
figure 8

TGA profiles of spent NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts

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Fig. 9
figure 9

DTG profiles of spent NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts

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3.7.3 TEM Analysis

Figure 10 displays the TEM images of the spent NiAl2O4, ZnAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts. TEM images of spent catalysts reveal notable differences compared to the fresh catalysts. All the used catalysts exhibit more agglomerated particles compared to fresh catalysts. This indicates that all the catalysts were resistive towards carbon formation during the catalytic reaction.

Fig. 10
figure 10

TEM images of spent a NiAl2O4, b ZnAl2O4, c NiO/ZnAl2O4 and d ZnO/NiAl2O4 catalysts

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4 Conclusion

CO2 hydrogenation over NiAl2O4, ZnAl2O4, NiO/ZnAl2O4, ZnO/NiAl2O4 and Al2O3 catalysts have been investigated and it was found that NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts exhibit high activity with CO2 conversion of 65% at 600 °C, which is several times more active compared to other catalysts reported in the literature. On the other hand, these catalysts showed a high methane selectivity even at high temperatures. The higher catalytic activity and CH4 selectivity of NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts can be attributed to the presence of metallic Ni under the reaction conditions which can enhance the CO2 hydrogenation activity.