Methanation of CO₂ on bulk Co–Fe catalysts

Highlights

• The efficiency of CO₂ methanation was estimated in the presence of Co–Fe catalysts.

• Dissociation of CO₂ molecules into discrete C and O atoms was confirmed.

• Fast production of CH₄ and slow H₂O formation was considered in Sabatier reaction.

• OH group from the FeOOH complex recombine in H₂O molecule using H atom at the surface.

Abstract

The efficiency of CO₂ methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO₂ methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO₂ molecule on the ensemble and detailed reaction mechanism of CO₂→CH₄ conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H₂O molecule was considered as the rate-limiting step.

Introduction

In the recent decades active consumption of fossil fuels has led to increased emission of carbon dioxide (CO₂) that constitutes one of the main components of greenhouse gases and an effective absorber of infrared radiation. Higher CO₂ concentration in the air directly affects global heat exchange and causes gradual climate change. The annual increase of CO₂ emission is calculated as 0.5–2 pmm that approximates 20 billion tons in absolute terms [[1], [2], [3]]. Developed technology to obtain fuel using biomass combustion might be considered to be “carbon-neutral technology” [[4], [5], [6]] when it does not increase the amount of CO₂ in the atmosphere. Production of CH₄ from atmospheric CO₂ might also be added to the category of “carbon-neutral technology” where both methane and water are “environmentally friendly” products [7]:

$$C{O}_2+4H_2\to C{H}_4+2H_{2}O,\Delta H^{298.15}=-164.7\frac{kJ}{mol}$$

At the beginning methanation was considered as a chemical approach to synthesize synthetic natural gas but a fast progress of hydrogen energy technologies during last 50 years shifted the emphasis on fuel towards pure hydrogen. Nevertheless, catalytic CO₂→CH₄ conversion on metal catalysts becomes more attractive because of a special interest in producing renewable fuel lowering global carbon dioxide emission. Discovered in 1897 methanation reaction (1) by the French chemists Paul Sabatier and Jean-Baptiste Senderens raises interest regarding effective CO₂→CH₄ conversion as well encourages further development of hydrogen technologies. From thermodynamical point of view, it would be a quite simple chemical process [[8], [9], [10], [11]]; however, very often it does not take place in practice due to kinetic limitations. Extraordinary stability of CO₂ molecules and steric factor when eight electrons are involved in the elementary act of interaction between CO₂ and H₂ is considered to be one of the obstacles. Therefore, the presence of a catalyst that can provoke and accelerate chemical reaction (1) would be appropriate [9]. It should be noted that the presence of a certain catalyst might result in a different reaction mechanism that itself leads to the appearance of other products or their mixtures (e.g. for (1) in addition to methane another hydrocarbon would appear [8,12,13]). However, in this paper we consider highly selective methanation catalysts only.

Experimental studies have confirmed that effective catalysts for reaction (1) might be metallic Rh [14,15], Ni [[16], [17], [18], [19]], Co [20,21] and Fe [[22], [23], [24]], but because of their disadvantages (Rh is very expensive, Ni and Co can be easily sintered and oxidized, Fe has low selectivity on methane) they were not developed. The use of bimetallic compounds allows preparing a catalyst that combines the desired properties of the individual metals. High activity of Co metal in the catalytic methanation [[25], [26], [27], [28]] is very well-known. Metallic Fe plays the role of a structuring component adding stability to sintering to the bimetallic compounds [29,30] and assists the CO₂ molecule splitting because of its high affinity for oxygen [31,32]. Multi-component alloys can demonstrate higher catalytic activity [23,27,30,[33], [34], [35], [36]] with respect to quantity and distribution of their components. All of these stimulate a systematic study of Co–Fe catalysts for chemical reaction (1).

In the present work catalyst efficiency of CO₂→CH₄ conversion was estimated using gas chromatography. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy were applied for ex-situ analysis of Co–Fe catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy (TPD-MS) experiments were performed to identify gaseous particles adsorbed on the surface of catalysts. Based on the experimental results, surface reaction model of CO₂ methanation on Co–Fe catalysts was proposed to understand regularities in the catalytic action.