Nickel phyllosilicate derived Ni/SiO₂ catalysts for CO₂ methanation: Identifying effect of silanol group concentration

Highlights

• Mesostructured silica nanoparticles with three different sizes were used as the sacrificial silica template.

• Silanol group concentration significantly affected the synthesis of nickel phyllosilicates.

• Nickel phyllosilicate catalysts were facilely prepared using hydrothermal method.

• Nickel phyllosilicate catalysts exhibited high activity and stability for CO₂ methanation.

• Nickel phyllosilicate catalysts exhibited highly hydrothermal stability in 100 % steam at 600 °C.

Abstract

For the conventional Ni/SiO₂ catalyst, the weak metal-support interaction usually resulted in serious Ni sintering at high temperature. To address this problem, a group of Ni-phyllosilicate-derived Ni/SiO₂ catalysts were synthesized via the hydrothermal reaction of nickel nitrate and mesostructured silica nanoparticles (MSN) with three different sizes of 99 (M_S), 184 (M_M), and 725 (M_L) nm. The Ni contents of three nickel phyllosilicate catalysts varied from 5.9 to 19.7 wt%, which exhibited a reverse order of the particle size. It was found that the concentration of silanol group (SiOH) of MSN significantly affected the construction of Ni-phyllosilicate material. The surface silanol group concentration increased with the decrease of particle size of MSN, resulting in the improved formation of nickel phyllosilicate and enhanced catalytic activity for CO₂ methanation. Among all catalysts, M_S/H-24 showed the highest CO₂ conversion of 76.5 % at 450 °C due to the highest Ni dispersion and largest CO₂ and H₂ uptakes. In addition, the optimal catalyst exhibited higher anti-sintering property, long-term stability and hydrothermal stability than an impregnated catalyst (Ms/Im) with the similar Ni content in a lifetime test (450 °C, 100 h) and steam treatment test (600 °C, 6 h), owing to its strong metal-support interaction derived from nickel phyllosilicate.

Introduction

Global warming derived from the massive emission of CO₂ has brought about tremendous challenge to the ecosystem, and these changes will directly affect the human living environment and sustainable economic development [1]. Achieving a “carbon peak” by 2030 and “carbon neutral” by 2060 has been a national strategy in China. CO₂ is not only a typical greenhouse gas to be eliminated, but also an important carbon-containing resource to be fully utilized [2,3]. Therefore, CO₂ capture, utilization and storage (CCUS) as an effective way to reduce greenhouse gases has attracted widespread attention [[4], [5], [6]]. It has been reported in the literatures that many chemicals including methane [7], synthesis gas [8], light olefins [9], ethers [10], etc. can be obtained through different catalytic reaction using CO₂ as a raw material. Compared with other CO₂ utilization routes, the synthetic natural gas (SNG) produced via CO₂ methanation reaction (CO₂ + 4H₂ → CH₄ + 2H₂O, ΔH_298K =–165 kJ mol^–1) with high activity and selectivity can be directly sent to the existed natural gas grids [7]. However, CO₂ is a very stable molecule, and the eight-electron reduction of CO₂ to CH₄ has significant kinetic limitations. Therefore, it is crucial to develop a CO₂ methanation catalyst with excellent catalytic performance.

Ni-based catalysts supported on various SiO₂ supports, namely Ni/SiO₂ catalysts, have been widely used in catalysis due to the advantages of large specific surface area, high cost performance and availability, etc [[11], [12], [13]]. For the Ni/SiO₂ catalyst prepared by the conventional impregnation method, Ni²⁺ ions are adsorbed and supported onto the SiO₂ support; however, the sintering of metallic Ni particles will generally occur at high temperature because of the weak interaction between nickel and SiO₂, resulting in reduced catalytic activity and poor stability of the catalyst [14].

To address the metal sintering problem of Ni/SiO₂ catalyst, many efficient strategies have been proposed. For intense, the modified preparation methods, including surfactant-assisted urea precipitation method [15], sol-gel method [16] and complexed-impregnation method [17], can improve the properties and morphology of Ni/SiO₂ catalysts. In addition, some promoters such as CeO₂ [18], La₂O₃ [19] and V₂O₅ [20] can not only effectively improve the Ni dispersion, but also alleviate the sintering problem due to the physical barrier effect. Furthermore, the morphology and porous structure of silica support has an important impact on catalytic performance of the Ni/SiO₂ catalyst. Encapsulating Ni particles inside the channels of the ordered mesoporous silica-based materials is an effective strategy to improve the stability and activity of the catalyst. For example, Syahida et al. [21] found that the nickel catalyst prepared using fibrous type SBA-15 as the support exhibited both higher catalytic activity and stability than the rod-like SBA-15 supported nickel catalyst. Polyhydroxy compounds (trehalose and ethylene glycol) were used as the sacrificial carbon template to prepare Ni/FDU-12 catalysts by an impregnation method, the formed carbon species in the inert flow could scatter around the Ni species and inhibit the sintering phenomenon [22]. Although the previously reported methods can improve the dispersion and anti-sintering performance of active species to a certain extent, their complicate preparation process and low Ni loading cannot meet the conditions for large-scale production and application of industrialized catalysts. Therefore, it is very important to design a Ni/SiO₂ catalyst with high Ni loading and excellent high temperature anti-sintering property.

Metal phyllosilicate materials can be converted to silica supported metal catalysts after reduction, which have been widely used in catalysis due to their advantages such as the unique microscopic morphology, tunable components and pore structure as well as strong metal-support interaction. For example, Kawi et al. [23] prepared copper phyllosilicate catalyst by ammonia evaporation method and applied it to water-gas shift reaction with high activity and stability. Ghiat et al. [24] prepared nickel phyllosilicate as a photo-catalyst for hydrogen generation by a hydrothermal method, and the prepared catalyst showed more effective separation and better transmission of photo-induced pairs (e⁻/h⁺). Nickel phyllosilicates can be classified two types (Ni₃Si₄O₁₀(OH)₂ and Ni₃Si₂O₅(OH)₄) according to Si/Ni ratio (2:1 or 1:1) (Fig. 1). Hydrothermal method is a widely used method for preparation of nickel phyllosilicate through the reaction of silica and nickel species. As reported, pH value of the mixed solution system can significantly affect the crystal form and crystallization properties of nickel phyllosilicate: (a) Only Ni(OH)₂ can be formed through the hydroxylation and polymerization reactions between Ni²⁺ ions and silanol groups on the surface of silica framework under neutral condition (Eq. 1); (b) Ni‒O‒Si heterogeneous polycondensation reaction occurs between Ni²⁺‒(OH)₂ and silicic acid (denoted as SiOH) on the SiO₂ framework to form 1:1 nickel phyllosilicate under alkaline conditions (Fig. 1, Eqs. 2 and 3); (c) More silicic acid will be formed under acidic conditions, and 2:1 nickel phyllosilicate can be obtained through the further reaction between 1:1 nickel phyllosilicate and silicic acid (SiOH) (Fig. 1, Eq. 4) [25]. Sivaiah et al. [26] prepared two nickel phyllosilicates (1:1 and 2:1) by hydrothermal method via changing the pH value of the solution for carbon dioxide reforming of methane, and the 2:1 nickel phyllosilicate showed higher thermal stability and catalytic performance. Thus, the unique property of nickel phyllosilicate (especially 2:1 type) with strong interaction between the active component and support, may be instrumental in improving the anti-sintering property of Ni/SiO₂ catalyst for CO₂ methanation.

Ni²⁺‒(OH)₂ + Ni²⁺‒(OH)₂ → Ni‒OH‒Ni

Ni²⁺‒(OH)₂ + OH– → Ni⁺‒OH + H₂O

Ni⁺‒OH + Si−OH + OH– → Si‒O‒Ni−OH + H₂O

Si‒O‒Ni−OH + Si−OH → Si‒O‒Ni‒O‒Si + H₂O

The morphology and structure of silica sacrificial templates determine the physical and chemical properties of nickel phyllosilicate. At present, zero-dimensional to three-dimensional silica-based materials with controllable morphology of SiO₂ materials have been widely used in catalysis [27], biosensors [28], batteries [29] and other fields [30]. The MSN materials obtain the characteristics of high specific surface area, adjustable, controllable pores and easy functionalization, which is widely used in the fields of adsorption [31], catalyst [32] and drug delivery [33]. Nanosize effect is of great significance to the design of nanomaterials [[34], [35], [36]], because the number of atoms present on the surface increases as the particle size of the silica-based material decreases [37]. Therefore, it is of great significance to explore the influence of the particle size effect as well as the derivative property change capability on the formation of nickel phyllosilicate.

In this work, three MSN materials with different particle sizes were prepared and a series of MSN-derived Ni-phyllosilicate catalysts with uniform, high-loaded and small-sized Ni particles were prepared by hydrothermal method for CO₂ methanation. The size effect and concentration of silanol group of MSN as well as hydrothermal time on the performance and structure of the formed nickel phyllosilicate catalyst were investigated. To the best of our knowledge, there is no report in the literature for investigation of effect of silanol group concentration on the formation and catalytic performance of nickel phyllosilicate for CO₂ methanation reaction. Catalytic activity, 100 h-lifetime test, hydrothermal stability measurement and various characterizations were performed to reveal the relationship between structure and catalytic performance.