Preparation of highly active and stable nanostructured Ni-Cr₂O₃ catalysts for hydrogen purification via CO₂ methanation reaction

Highlights

• A novel, simple, and solvent-free synthesis method was used for the catalyst synthesis.

• The amount of nickel loading affected the catalytic performance.

• 20 wt%Ni/Cr₂O₃ showed high activity and selectivity in methanation of CO₂.

• 20 wt%Ni-Cr₂O₃ mixed oxide catalyst possessed higher activity than the 20 wt%Ni/Cr₂O₃.

Abstract

In this study, a series of Ni/Cr₂O₃ catalysts with various Ni percentages were synthesized using a solvent-free mechanochemical method and the prepared catalysts were evaluated in CO₂ methanation reaction. The influence of metal loading on the catalytic performance was investigated and it was found that increasing in Ni content improved both the carbon dioxide conversion and CH₄ selectivity due to the increase in the concentration of active sites. It was evident that among the samples with different Ni loadings, the 20 wt%Ni/Cr₂O₃ catalyst demonstrated the highest catalytic performance. The catalytic performance of the Ni-Cr₂O₃ mixed oxide was also compared with Ni/Cr₂O₃ catalyst. The Ni-Cr₂O₃ mixed oxide catalyst was more active than the Ni/Cr₂O₃ catalyst and exhibited 68.98% CO₂ conversion and 100% CH₄ selectivity at 350 °C (H₂/CO₂ = 3 M ratio, GHSV = 18000 ml/g_cat.h). The higher activity of the mixed oxide catalyst was due to the higher specific surface area, smaller crystalline size, and improved reducibility. Also, 20 wt%Ni-Cr₂O₃ composite catalyst exhibited high stability for 12 h at 300 °C in the CO₂ methanation reaction. It was seen that the increase in H₂/CO₂ molar ratio from 2 to 5 and also the decrease in GHSV value and calcination temperature improved the catalytic activity.

Introduction

The utilization of fossil fuels leads to increased CO₂ emissions in the atmosphere, which is problematic to the global greenhouse effect [1,2]. Turning this greenhouse Gas into Valuable Products can be considered as an interesting solution to reduce the negative effect of this greenhouse gas on the atmosphere. CO₂ hydrogenation can create a wide range of chemicals such as hydrocarbons, higher alcohols, and liquid fuels. However, among these reactions CO₂ methanation is an important and famous reaction (Eq. (1)) [[3], [4], [5]]:

CO₂ + 4H₂ ↔ CH₄ + 2H₂O ΔH _298K = −164 kJ/mol

The CO₂ methanation is an exothermic reaction. Since, the methanation of carbon dioxide is a kinetic limited reaction, the use of a highly active catalyst is necessary for industrial applications [[6], [7], [8], [9]]. The key challenge in CO₂ methanation is the development of highly efficient and low-cost catalysts. In recent years, different catalysts have been studied for this reaction. The researchers found that CO₂ methanation can be performed on the VIII group metals like Ru, Ir, Rh, Pd, Pt, Ni, Co, Fe supported on various oxide supports (such as Al₂O₃, SiO₂, ZrO₂, CeO₂, TiO₂, La₂O₃, MgO) [1,[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. Among these catalysts, the Ni-based catalysts are the most widely studied in commercial methanation processes because of their high activity, low cost, and high availability compared to noble metal catalysts [[26], [27], [28], [29], [30]]. It is well known that the type of the catalyst carrier can dramatically affect the interaction between the Ni and the carrier, which ultimately affects the catalytic performance in methanation of CO₂ [22,31]. Chromium oxide is one of the most promising candidates for catalytic reactions such as polymerization, dehydrogenation, dehydrocyclization, etc. [32]. For example, Landau et al. [33] reported that chromium possessed higher activity compared to the other transition metal oxides for the oxidation of n-butane and ethyl acetate. More recently, we reported that Cr₂O₃ supported catalysts were active for CO₂ methanation. We found that Ni-based catalysts supported on Cr₂O₃ showed good catalytic performance compared to Al₂O₃ supported catalysts. It is known that several synthesis methods have been used to synthesize CO₂ methanation catalysts, such as precipitation [34], sol-gel [35], hydrothermal [36], impregnation [37], etc. Sepehri et al. [38] used a solid-state synthesis method and reported that the catalysts prepared with this method showed high activity in methane autothermal reforming. In the mechanochemical synthesis method, there is no solvent, therefore there is no requirement to remove the solvent at the end of the synthesis. This mechanochemical synthesis method compared to the conventional synthesis methods is a simple, and facile method, which can be performed at ambient temperature. In this solvent free method, waste generation has been minimized, and the method can be mentioned as an environmental friendly method. Ammonium carbonate acts as a precipitating agent. By releasing the hydrate water of metal salt precursors and the formation of pasty intermediate, basic metal carbonates were created [[39], [40], [41], [42]]. It is known that the loading of nickel affects the physicochemical and catalytic performance of the catalysts in the methanation reaction [43,44].

In this study, we investigated the effect of Ni content on the catalytic performance of the nickel catalysts supported on Cr₂O₃. Also, NiO-Cr₂O₃ mixed oxide catalyst was prepared and evaluated in this reaction.