Impacts of La addition on formation of the reaction intermediates over alumina and silica supported nickel catalysts in methanation of CO2

https://doi.org/10.1016/j.joei.2019.05.009Get rights and content

Highlights

  • La addition to Ni/Al2O3 or Ni/SiO2 reduced Ni size, NiO reduction while increased alkalinity.

  • Ni/SiO2 was much more active than Ni/Al2O3, even though Ni size was much bigger.

  • La addition improved catalytic activity of the two catalysts and suppress CO formation over La-Ni/SiO2.

  • La addition to Ni/Al2O3 inhibited *CO3 or HCO3*, while favored HCOO* formation, enhancing catalytic activity.

  • CO* was main reaction intermediate over Ni/SiO2 and La addition facilitated its hydrogenation to CH4.

Abstract

This study aimed to investigate impacts of Al2O3 and SiO2, the supports of Ni catalysts with distinct properties, and the additive of La on catalytic behaviors and reaction intermediates formed during methanation of CO2. The results showed that the addition of La to either Ni/Al2O3 or Ni/SiO2 led to the reduced size of metallic nickel, the reduced reduction degree of nickel oxide, the increased alkalinity number and the increased activity for methanation of CO2. Furthermore, the addition of La to the Ni/SiO2 catalyst could suppress the formation of CO via the reverse water gas shift (RWGS) reaction. Ni/SiO2 was much more active than the Ni/Al2O3, even though nickel size was much bigger. The in situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies showed that the addition of La to Ni/Al2O3 interfered with integration of hydroxyl group with *CO2 species and formation of the bicarbonate and carbonate, while favored formation of the formate specie, enhancing the catalytic activity. For Ni/SiO2, instead of formate, CO* became the main reaction intermediate. The strong absorption of CO* favored its further conversion and explained the low selectivity of the silica-based catalysts toward CO. The addition of La to Ni/SiO2 catalyst facilitated further hydrogenation of CO* species to CH4 and promoted the catalytic activity.

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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