Ni–Zn–Al-Based Oxide/Spinel Nanostructures for High Performance, Methane-Selective CO2 Hydrogenation Reactions

In the present study, NiO modified ZnAl2O4 and ZnO modified NiAl2O4 spinel along with pure Al2O3, ZnAl2O4 and NiAl2O4 for comparison in the CO2 hydrogenation reaction have been investigated. It was found that NiAl2O4, NiO/ZnAl2O4 and ZnO/NiAl2O4 catalysts exhibited outstanding activity and selectivity towards methane even at high temperature compared to similar spinel structures reported in the literature. 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. The high activity and selectivity of methane can be attributed to the presence of metallic Ni and Ni/NiO/ZnAl2O4 interface under the reaction conditions as evidenced by the XRD results. High performance Ni–Zn–Al-based oxide/spinel nanostructures is synthesized and NiO/ZnAl2O4 catalyst exhibited higher catalytic activity in the CO2 hydrogenation reaction due to the presence of metal support interaction between Ni and ZnAl2O4 support.


Introduction
The catalytic conversion of CO 2 is desirable strategy to not only reduce the CO 2 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 (CO 2 + H 2 ↔ CO + H 2 O, ΔH RWGS = + 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 (CO 2 + 4H 2 → CH 4 + 2H 2 O, ΔH Sab = − 165 kJ/mol) and thermodynamically favoured at low temperatures [10]. Ni [11], Ru [4] and Rh [12] are most widely used catalysts for CO 2 methanation reaction. Cu [13] and Pd [14] are most widely used catalysts for the reduction of CO 2 to methanol [15][16][17]. Nickel based catalysts have been widely investigated as catalyst in CO 2 hydrogenation reactions owing to its superior catalytic activity and low cost [18,19]. Recently, nickel based spinel catalysts have been widely used in CO 2 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, NiAl 2 O 4 was found to minimize the coke formation in CO 2 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 CO 2 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, N 2 physisorption, TEM, SEM-EDX and TGA. These catalysts were tested in CO 2 hydrogenation reaction in the gas phase. It was found that NiAl 2 O 4 , NiO/ZnAl 2 O 4 and ZnO/NiAl 2 O 4 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/ZnAl 2 O 4 catalyst showed CO 2 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.

Catalyst Preparation
The ZnAl 2 O 4 oxide was synthesized by a co-precipitation method in accordance with the procedure reported in the previous work [34]. Typically, appropriate amount of Zn(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 ·9H 2 O 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.

N 2 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 N 2 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.

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°.

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 .

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.

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.

Hydrogenation of Carbon-dioxide in a Continuous Flow Reactor
Before the catalytic experiments, the as-received catalysts were oxidized in O 2 atmosphere at 300 °C for 30 min and thereafter were reduced in H 2 at 300 °C for 60 min. Catalytic reactions were carried out at atmospheric pressure in a fixedbed 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 CO 2 :H 2 molar ratio was 1:4, if not denoted otherwise. The CO 2 :H 2 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 CO 2 was transformed by a methanizer to methane and it was also analysed by FID. CO 2 conversion was calculated on a carbon atom basis, i.e. CH 4 selectivity and CO selectivity were calculated as following where CO 2 inlet and CO 2 outlet represent the CO 2 concentration in the feed and effluent, respectively, and CH 4 outlet and CO outlet represent the concentration of CH 4 and CO in the effluent, respectively.

X-ray Diffraction (XRD)
The crystal structure of catalysts was investigated by XRD. Figure 1 shows

N 2 Adsorption-Desorption Isotherm
The specific surface area together with the pore volume and pore size was summarized in Table 1. The N 2 adsorption-desorption isotherms of ZnO/NiAl 2 O 4 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, Al 2 O 3 , NiAl 2 O 4 , ZnAl 2 O 4 and NiO/ZnAl 2 O 4 displays type IV isotherms with H2 hysteresis loop at P/ P 0 = 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 NiAl 2 O 4 and ZnAl 2 O 4 , the resulting catalyst showed decreased surface area and pore volume.

TEM Analysis
The morphology and particle size of the catalysts were examined by TEM measurements and shown in Fig. 2

Catalytic Performances
To explore the catalytic performance, CO 2 hydrogenation was performed over the prepared catalysts. Figure 3 depicts the CO 2 conversion as a function of temperature over all the catalysts. CO 2 conversion and product selectivity are given in Table 3    (Conversion = 23%) and twofold superior in catalytic activity than that of ZnAl 2 O 4 (Conversion = 31%). 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 ( This can be correlated with increasing Ni content. Given that an increase in Ni content can enhance CO 2 hydrogenation activity [40]. The NiO/ZnAl 2 O 4 exhibited 65% CO 2 conversion at 600 °C with CH 4 and CO as the products. All of the Ni containing catalysts produce CH 4 as main products and CO as minor products while ZnO and other Zn containing catalysts as well as Al 2 O 3 produce only CO.  In general, Ni based catalysts produce CH 4 through decomposition of formate species to CO and subsequent hydrogenation of adsorbed CO leads to the production of CH 4 [41] and ZnO is more active for the RWGS reaction [42]. Table 4 lists the CO 2 consumption rates of all the catalysts studied at 600 °C. Figure 5 depicts the CO 2 consumption rate as a function of temperature for all the studied catalysts. The CO 2 consumption rate is highest on NiO/ZnAl 2 O 4 , namely ca. 19.7 μmol h −1 g −1 at 600 °C which was 2.5 times higher than that of Al 2 O 3 (ca. 7.9 μmol h −1 g −1 at 600 °C) catalyst. This catalyst also outperforms other reported spinel catalysts (Table 5) in the CO 2 hydrogenation reaction.
Although the surface area of Al 2 O 3 was far higher than the NiO/ZnAl 2 O 4 , the CO 2 consumption rate was far higher on NiO/ZnAl 2 O 4 . This was due to presence of metallic Ni under reaction condition in NiO/ZnAl 2 O 4 than in the other catalysts. Comparative table of CO 2 consumption rate of the catalyst in this study with the spinel catalyst reported in the literature for CO 2 hydrogenation is given in Table 5.
The effect of metal-support interaction was investigated over Ni/SiO 2 catalyst in the CO 2 hydrogenation reaction 20 [43]. It was reported that the oxygen vacancy present in the support produces surface carbon species and Ni dissociates H 2 into atomic hydrogen [44]. In the present study, the high catalytic activity of NiO/ZnAl 2 O 4 catalyst can be attributed to the strong interaction between the Ni and the ZnAl 2 O 4 leading to the incorporation of Ni into the ZnAl 2 O 4 lattice and subsequent formation of oxygen vacancies [45]. This oxygen vacancies produce surface carbon species and the Ni dissociates H 2 into atomic hydrogen and forms CO and CH 4 as the final products. Figure 6 shows the stability test of all catalysts for CO 2 hydrogenation. For all the catalysts, CO 2 consumption rate had no obvious decline with time. This suggested that all the catalysts are more stable during CO 2 hydrogenation reaction. The ZnO/NiAl 2 O 4 catalyst showed excellent catalytic stability for CO 2 hydrogenation among all the catalysts studied.

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

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 [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 effectively reduce carbon deposit. This is in line with their higher catalytic activity in CO 2 hydrogenation reaction ( Table 4).