Synthesis of Ni₅Ga₃ catalyst by Hydrotalcite-like compound (HTlc) precursors for CO₂ hydrogenation to methanol

Highlights

• Highly pure Ni₅Ga₃ was synthesized via Hydrotalcite-like compound precursor.

• The method was highly selective to synthesize Ni₅Ga₃ bimetallic catalyst.

• The precursor structure was crucial for the reduced Ni₅Ga₃ structure.

• The as-prepared catalysts exhibited high methanol selectivity.

Abstract

A Ni₅Ga₃ catalyst synthesized from hydrotalcite-like compound (HTlc) precursors was examined for CO₂ hydrogenation to methanol. HTlc precursors with a nickel atomic percentage of 65 % balanced with gallium were prepared by a urea hydrolysis hydrothermal method, and fully characterized. The HTlc phase in the nickel–gallium precipitant became better crystallized and the structure became more stable as the synthesis reaction temperature increased. Bimetallic alloy Ni₅Ga₃ was obtained by reducing the as-prepared HTlc precursors in a flow of 5 % H₂ balanced with Ar at a temperature of 700 °C. Ni–Ga HTlc precursor prepared at a hydrothermal temperature of 110 °C resulted in the formation of bimetallic alloy, Ni₅Ga₃, which produces smaller crystal size and a stable structure. Enhanced catalytic performance was demonstrated by an endurance test with a constant CO₂ conversion and 100 % methanol selectivity at 200 °C, and a turnover frequency of 0.27 s⁻¹.

Introduction

It is widely accepted that increasing anthropogenic emissions of CO₂ to the atmosphere is the major contributing factor to global warming. As CO₂ is a ubiquitous and cheap carbon source, utilisation of CO₂ has been a popular research activity over the last decade. Catalytic CO₂ hydrogenation to essential chemicals is one of many options to minimize greenhouse gas emissions and offers a promising and environmentally-friendly pathway to synthesize value-added chemicals such as methanol. Methanol is a primary material for producing subsequent valuable chemicals such as acetic acid, tert-butyl ether (MTBE) and chloromethane [1], and can also be used directly as a fuel for internal combustion engines [2,3].

The major routes for CO₂ hydrogenation involved in methanol synthesis are: (1) the direct hydrogenation of CO₂ to methanol, (2) the reverse water gas shift (rWGS), and (3) CO₂ methanation, as shown below in reactions (1), (2), (3), respectively. The rWGS is endothermic and is therefore favoured as temperature increases. On the other hand, both methanol synthesis and CO₂ methanation are exothermic and hence show a decline in equilibrium CO₂ conversion as temperature increases.

$$\begin{array}{cc}\hfill 3{\text{H}}_2+\text{C}{\text{O}}_2\rightleftharpoons \text{C}{\text{H}}_3\text{OH}+{\text{H}}_2\text{O}\hfill & \hfill \text{Δ}{\text{H}}_{298\text{K}}=-49.16\text{kJ}/\text{mol}\hfill \end{array}$$

$$\begin{array}{cc}\hfill {\text{H}}_2+\text{C}{\text{O}}_2\rightleftharpoons \text{CO}+{\text{H}}_2\text{O}\hfill & \hfill \text{Δ}{\text{H}}_{298\text{K}}=+41.21\text{kJ}/\text{mol}\hfill \end{array}$$

$$\begin{array}{cc}\hfill \text{C}{\text{O}}_2+4{\text{H}}_2\rightleftharpoons \text{C}{\text{H}}_4+2{\text{H}}_2\text{O}\hfill & \hfill \text{Δ}{\text{H}}_{298\text{K}}=-165.0\text{kJ}/\text{mol}\hfill \end{array}$$

The conventional, commercial catalyst for methanol synthesis from syngas is based on copper (Cu), and is carried out in the pressure range of 5−10 MPa and in the temperature range of 250−280 °C [2]. Despite their commercial success, these catalysts still need improvement, particularly with respect to low selectivity of methanol and fast deactivation of active copper sites [4]. To improve the catalytic performance, it has become common practice to incorporate metal oxide additives such as ZrO₂, Ga₂O₃ and TiO₂ into the copper-based catalyst, which serve to increase its thermal stability and prevent sintering of active copper [[5], [6], [7], [8]]. Another approach towards an improved catalyst is to abandon the Cu based system completely and pursue the alternative alloys. For example, bimetallic catalysts such as nickel–gallium [9] and palladium–gallium [10] have been developed, and these catalysts are especially advantageous in CO₂ hydrogenation to methanol under milder temperature and pressure conditions.

Ni–Ga bimetallic catalysts were investigated regarding the active sites for CO₂ reduction to methanol [9]. Density functional theory (DFT) calculations revealed that the theoretical activity of Ni₃Ga and Ni₅Ga₃ were close to Cu–Zn commercial catalysts. To further investigate the catalytic performance of the Ni–Ga catalyst system, different Ni–Ga phases, such as Ni₃Ga, Ni₅Ga₃ and NiGa were tested for CO₂ hydrogenation to methanol. Among them Ni₅Ga₃ showed similar CO₂ conversion with a high methanol selectivity compared with the commercial copper-based catalysts. Ni₃Ga, however, showed a significant amount of methane, resulting in low methanol selectivity compared with Ni₅Ga₃ [9]. The high methanol selectivity was due to the nickel-rich Ni₅Ga₃ active sites being poisoned by CO generated from the rWGS reaction, which hindered the rWGS reaction and in turn facilitated the methanol synthesis. Further investigation on Ni₅Ga₃ catalyst was conducted recently [11]. A Ni–Ga alloy prepared by a co-condensation method showed high selectivity for methanol (90 %) with slower deactivation of the catalyst [11]. Nitoslawski et al. [12] reported that an amorphous Ga₂O₃ shell around Ni₅Ga₃ could further promote methanol synthesis.

The purity of Ni₅Ga₃ catalyst prepared with the impregnation method reported above, however, was limited and the catalyst easily deactivated. Ni₅Ga₃ catalysts were synthesized by Ahmad and Upadhyayula [13] via co-condensation–evaporation, incipient wetness impregnation and also the co-precipitation method, but the purity of Ni₅Ga₃ was restricted in all reduced samples according to the corresponding PXRD pattern analysis. Additionally, the nickel to gallium ratio to prepare Ni₅Ga₃ is limited to a specific range, which prohibits synthesis of pure Ni₅Ga₃ [14]. Furthermore, the co-condensation method is relatively complicated for synthesizing the target Ni₅Ga₃ alloy catalyst.

Hydrotalcite-like compounds (HTlc) have been used as a precursor to produce catalysts and offer a number of advantages such as, producing well dispersed active sites, a uniform distribution of metal elements in the catalyst, and correspondingly improving catalytic performance. This method has therefore been widely adopted in the preparation of copper [[15], [16], [17]] and nickel based catalysts [16,17] for CO₂ hydrogenation to methanol and methane. Alloys, such as Ni–Fe, Co–Fe and Ni–Ir prepared from HTlc also offer excellent catalytic performance, demonstrating a strong resistance to coking and enhanced activity [[18], [19], [20]].

The structure of HTlc can be represented as [M_1-x²⁺M_x³⁺(OH)₂]A_x/nⁿ⁻·mH₂O, where M²⁺ and M³⁺ represent divalent and trivalent cations, respectively, and include Mg²⁺, Ni²⁺, Cu²⁺, Co²⁺ or Mn²⁺ and Al³⁺, Cr³⁺ or Ga³⁺ [21], where x normally varies between 0.2 and 0.4 but pure phases usually form at 0.2 ≤ x ≤ 0.33 [21].The cations in HTlc are trapped into the hydroxide octahedral framework, and the octahedral units display a layered structure, which is the character of a two-dimensional HTlc material. Aⁿ⁻ are anions located between the interlayers, neutralized by extra positive charge from M³⁺ in octahedral layers. HTlcs are usually prepared by co-precipitation under a constant pH and hydrothermal synthesis. The advantage of hydrothermal synthesis, especially controlled by the urea hydrolysis reaction, is that it offers more opportunities to produce mono-dispersed particles because the mild alkaline environment favors the slow and uniform nucleation of the HTlc [22]. Thus, bimetallic Ni₅Ga₃ catalyst, which is difficult to synthesize with a co-precipitation method, can in principle be efficiently and readily produced by a hydrothermal method, as an alternative method to prepare Ni₅Ga₃.

We report here the preparation method of Ni₅Ga₃ bimetallic catalysts derived from Ni–Ga HTlc precursors and their subsequent performance in CO₂ hydrogenation. The synthesized Ni–Ga HTlc precursors and as-prepared Ni₅Ga₃ samples were characterized by powder X-ray diffraction (PXRD), thermal gravimetric analysis (TG), scanning electron microscope (SEM) and X-ray absorption spectroscopy (XAS). The impact of synthesis reaction temperature in the hydrothermal treatment process on HTlc structure was investigated, and the relationship between the precursor structure and the property of the as-obtained Ni₅Ga₃ crystal was analyzed. The optimal reaction temperature in the hydrothermal treatment process was consequently determined. The catalytic performance of Ni₅Ga₃ catalyst prepared by HTlc precursors was also evaluated in CO₂ hydrogenation to methanol.