Impacts of La addition on formation of the reaction intermediates over alumina and silica supported nickel catalysts in methanation of CO2
Graphical abstract
Introduction
Methanation of carbon dioxide is known as the Sabatier reaction: CO2 + 4H2 → CH4 + 2H2O ΔH298k = −165 kJ mol−1 [1], [2], via which the CO2 produced from combustion of carbon resources can be utilized and recycled. Further to this, the Sabatier reaction is also used as a method for the production of synthetic natural gas (SNG) for civil use [3], [4]. One of the biggest challenges in the process is the development of the highly active and stabile catalysts for methanation of CO2. Methanation requires the full hydrogenation of the C=O bonds in CO2 with hydrogen, which involves the activation of both CO2 and hydrogen on surface of catalyst [5]. To activate both CO2 and hydrogen is a challenge for many catalysts. In addition, methanation of CO2 is an exothermic reaction, which may lead to sintering of metallic phase [6], [7]. Further to this, methanation of CO2 involves the formation of the reaction intermediates such as CHx, C=O*, HCOO* species, which might not be effectively gasified and then could condense to coke. The CO formed via the reverse water gas shift (RWGS) reaction could also be a precursor for coke formation via the Boudouard reaction, especially in the mild temperature regions [8], [9], [10].
To tackle the coking issue and to enhance the efficiency for methanation, much effort has been made to develop catalysts with desirable activity and stability [11]. The existing literature indicate that the precious metals (Pd, Rh, Ru) [4], [12], [13], [14] and the transition metals (Fe, Co, Ni) based catalysts [15], [16], [17] are active for methanation of CO2. The mono metals, however, generally have low specific area and needs to be supported and dispersed in a porous support to enhance the utilization of the active metals [18], [19]. The metal oxides such as Al2O3 [20], [21], SiO2 [22], [23], TiO2 [24], [25], ZrO2 [26], [27] and molecular sieve [28], [29], [30] have varied acidities/basicity, which could potentially involve in methanation of CO2 via impacting the absorption/activation of CO2. To modify the surface properties of these oxide-based supports, additives such as the alkali metals [31], [32], alkaline earth metals [33], [34] and the rare earth metals (La, Ce) [35], [36], etc have been employed. The additives could affect the interaction of the active metals with support, the reduction behaviors of oxide form of the active metals, the acidity of support, etc, which has been the focus for numerous studies about the methanation of CO2 [37], [38]. Much less attention has been paid to understand how the additives impact the reaction intermediates formed during methanation of CO2. Understanding the roles of the additives on the reaction intermediates formed during methanation of CO2 could help to understand the effects of an additive on performance of a catalyst from the angle of fundamental aspect.
To understand how an additive affect the reactive intermediates formed during CO2 methanation, the frequently used nickel catalysts (Ni/Al2O3 and Ni/SiO2) were selected as the model catalysts, while lanthanum (La) was used as a model additive. Lanthanum, as a rare earth element, has unique properties, and the influence of La addition on the catalytic activity of nickel catalysts has been reported by other researchers [36], [39]. For example, lanthanum oxide could react with CO2 at elevated temperature, enhancing efficiency for gasification of carbonaceous species with CO2 in steam reforming reactions [36], [39]. In this study, we focused particularly on the impacts of La on the formation of the intermediates by using an in-situ DRIFTS characterization of the reaction intermediates formed during the methanation of CO2. The results indicated the drastically different effects of La on the physiochemical properties of the catalysts and the reactive intermediates formed during the methanation of CO2.
Section snippets
Catalyst preparation
The catalysts were prepared by an incipient wet impregnation method. The loading of La2O3 to Al2O3 or SiO2 in terms of weight was 0.1, 0.5, 1, 5 and 7.5 wt%, respectively, while the loading of the metallic nickel to Al2O3 or SiO2 was 15 wt%. The catalysts were prepared via the following procedures. Firstly, La(NO3)3 solution with a certain concentration was added to alumina or silicon for the impregnation. The impregnated samples were then dried at room temperature for 12 h and then dried in a
N2 adsorption analysis
The N2 physical adsorption-desorption isotherm of the catalysts is shown in Fig. 1. It can be seen that the shape of the isotherm showed the characteristic feature of a typical mesoporous material when alumina was used as the support (Fig. 1a). The samples all showed a representative IV(a) isotherm having a H1 shape hysteresis loop. The addition of La did not change the porous structure of the support. The N2 physical desorption isotherm for the catalysts using silica as a support exhibited the
Conclusions
In summary, the alumina or silica supported nickel catalysts had distinct physiochemical properties and the addition of La significantly impacted the catalytic performances and the formation of the reaction intermediates in CO2 methanation. For the alumina-based catalysts, the addition of La did not filled much of the pores in alumina and the aggregation of metallic nickel particles could be suppressed. The particle size of nickel over the Ni/Al2O3 catalyst was only half to that over the Ni/SiO2
Acknowledgements
This work was supported by the Strategic International Scientific and Technological Innovation Cooperation Special Funds of National Key R&D Program of China (No. 2016YFE0204000), the Program for Taishan Scholars of Shandong Province Government, the Recruitment Program of Global Young Experts (Thousand Youth Talents Plan) and Natural Science Fund of Shandong Province (ZR2017BB002).
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2022, FuelCitation Excerpt :These results agreed with the previous work reported by Li et al., suggesting that incorporating Mg into Ni-based catalyst significantly affected the basic concentration and basic strength of the catalyst [31]. Another publication also reported that the CO2 methanation correlated to the level of medium basic sites, as the adsorption of CO2 over weak basic sites was not sufficient to break the C-O bond [32]. Thus, the increased level of moderately basic sites in the Ni-based catalysts revealed a high catalytic performance in CO2 methanation.