Influence of metal loading and reduction temperature on the performance of mesoporous NiO–MgO–SiO₂ catalyst in propane steam reforming

Highlights

• NiO–MgO–SiO₂ was used as a novel mesoporous catalyst for the syngas production.

• Optimum Ni content leading to the most active catalyst was determined to be 15 wt%.

• The S/C of up to 4 enhanced the C₃H₈ conversion (95%) and H₂ yield (82%) at 650 °C.

• Reduced sample at 550 possessed the highest dispersion of 8.8%.

Abstract

In this research, a series of NiO–MgO–SiO₂ catalyst samples with various nickel contents (5, 10, 15 and 20 wt %) were prepared by a co-precipitation method followed by a hydrothermal treatment and employed in propane steam reforming. The analyses revealed that the enhancement of the nickel content up to 15 wt % improved the propane conversion to 98.6% at 550 °C. Nonetheless, further increase in the nickel loading reduced the catalyst activity due to the formation of larger and more poorly dispersed active sites. Besides, 15 wt % nickel loading led to the high resistance against coke deposition with no detectable carbon on the catalyst surface. In addition, it was revealed that, the decrease in steam to carbon (S/C) ratio caused further carbon depositions upon the catalyst surface as well as producing higher extents of undesired by-products. Besides, results proved that the alternation of reduction temperature within 500–650 °C caused a significant influence on the dispersion and particle size of metallic Ni. The accessible metallic Ni for reactants and consequently catalyst performance was strongly affected by reduction temperature. The results exhibited that the reduced catalyst at 550 °C possessed the highest activity with the lowest amount of by-products owning to the formation of highly dispersed small Ni species. Higher reduction temperatures accelerated the agglomeration of Ni species, which negatively affected the catalytic performance.

Introduction

Nowadays syngas production, with CO and H₂ as the principal constituents has received a good deal of attention as an important research topic in catalysis. Hydrogen and syngas are major building blocks of many chemical compounds as well as proper feedstock for the manufacture of various chemicals for instance methanol and ammonia. Moreover, hydrogen as a renewable clean source of energy has attracted a great deal of attention as a legitimate alternative for fossil fuels [1]. As for various technologies for syngas production, catalytic steam reforming of hydrocarbons is considered as conventional route from both industrial and commercial viewpoints. In particular, propane as the main component of liquefied petroleum gas (LPG), being produced as a by-product of natural gas processing and petroleum refining, seems to be a potential feed candidate for catalytic steam reforming process. In fact, propane fuel as a hydrogen source possessed many advantages including high energy density, well-developed infrastructure with widespread distribution network and easy transportation. In addition, for the fuel cell application, propane is considered to be an appropriate feed for small scale steam reformers [2,3]. The propane steam reforming contains three main reactions including the i) highly endothermic propane steam reforming, ii) water gas shift, and iii) Boudouard reactions [4] i.e.

$${\text{C}}_3{\text{H}}_8+3{\text{H}}_2\text{O}\leftrightarrow 3\text{CO}+7{\text{H}}_2{\text{H}}_{298}^{°}=499\text{ kJ}/\text{mol}$$

$$\text{CO}+{\text{H}}_2\text{O}\leftrightarrow {\text{CO}}_2+{\text{H}}_2{\text{H}}_{298}^{°}=-41\text{ kJ}/\text{mol}$$

$$2\text{CO}\leftrightarrow \text{C}+{\text{CO}}_2{\text{H}}_{298}^{°}=-172\text{ kJ}/\text{mol}$$

On the other hand, Ni-based catalysts are widely applied in steam reforming, owing to their low price and reasonable activity. However, these materials suffer from coke deposition and sintering as two major deactivation problems. The coke formation is caused by two main reactions of CO disproportionation and hydrocarbon decomposition, reducing the catalyst activity via pore blockage and active site coverage. In addition, small Ni species have a tendency of minimizing their surface energy at high temperatures. This leads to agglomerating toward larger particle size as well as lowering dispersion and reducing active sites [5]. Consequently, much efforts have been devoted toward developing novel preparation and/or promotion techniques leading to Ni-based catalysts with considerable resistance against sintering and coke deposition. Many researches available in the literature, focused upon adding of highly reducible precious metals including Pd [6], Pt [7], Rh [8,9] and Ru [10], to provide the role of an active phase and/or a promoter [11]. It was revealed that, the presence of these noble metals in catalyst structure significantly enhanced the hydrogen selectivity and catalyst activity as well as reduced the coke deposition on the surface of catalysts. Nevertheless, the high cost and limited accessibility has restricted their industrially commercial utilizations. Moreover, the effect of La, Ce and Zr addition as oxygen carriers have been studied [12,13]. Natesakhawat et al. [14] have investigated effects of lanthanide elements series (La, Ce, and Yb) as promoters upon the Ni/Al₂O₃ catalytic performance and deactivation throughout propane steam reforming. They reported that lanthanides improved the sintering resistance while inhibiting the coke formation. These were attributed to enhancement of oxygen species becoming available through acceleration of water gasification and adsorption. Furthermore, modification of Ni based catalyst with alkaline and rare alkaline metals, such as Mg and Ca [15] have been examined. Preparation of bi- or tri-metallic [[16], [17], [18]], different combinations of metal oxides with well-defined structures such as MgAl₂O₄ [19], FeCe₂O₄ [20] and NiFe/Al₂O₃ [21] and employment of various preparation methods [22,23] were also considered as alternatives for developing catalysts highly resistant against sintering and coke deposition. In spite of various studies focusing on the deactivation of Ni based catalysts, coke formation is also considered as a major challenge encountered during steam reforming.

In general, two main factors may affect the carbon deposition on the surface of catalysts including, i) acid-base properties of the support and ii) size of the metal particles. In nickel-based catalysts, smaller Ni particles reduce the chance of coke deposition, because the coke formation can be restricted over the Ni crystals with smaller size than the critical size necessary for coke formation Furthermore, at a constant metal loading, smaller metallic particles can provide more accessible active sites for adsorption and reaction compared to those with larger size.

It is known that the use of special metals has a major influence on the catalyst properties such as specific surface area, metal-support interaction and particle size, which in turn affects the catalytic performance [[24], [25], [26]]. Wang et al. [27] studied the effect of Ni content in Ni/MgO catalyst from 5 to 20 wt% on the ethanol conversion and product yield. They found that all Ni loadings except 5 wt % resulted into complete conversion of ethanol. Increasing the Ni content led to the gradually increase in hydrogen yield, while the formation of methane declined. It was declared by An et al. [28] that the increase in Ni content of NiO/Al₂O₃ catalyst promoted the catalytic cracking reaction during steam reforming of acetic acid. They also investigated that coke deposition affected by Ni content in such a way that increasing the Ni loading from 9 to 15 wt % reduced the amount of Carbidic-like carbon. Khajenoori et al. [29] examined the influence of Ni loading on the catalytic behavior of NiO–CeO₂/MgO during the dry reforming of methane. They observed that the increase in Ni loading up to 10 wt % caused the increment of CH₄ and CO₂ conversion. Ni content higher than 10 wt %, negatively affected the catalytic behavior due to the formation of larger Ni crystals. Hence, according to the open literature devoted to the effect of metal loading, the sufficient metal loading seems to be necessary to control the active metal dispersion and particle size, which directly affects the catalyst activity and coke formation.

Another effective factor on the catalyst performance is the reduction temperature. Metal dispersion, active phase particle size and reduction degree are three main parameters, which can be modulated by reduction temperature. Marin et al. [30] recently investigated the performance of nickel aluminate catalysts reduced at different temperatures in glycerol reforming. In spite of slight alternation of specific surface area and average pore sizes, glycerol conversion and the distribution of liquid phase products were considerably affected by reduction temperature. They observed that by increasing the reduction temperature, the accessible metallic Ni surface was increased, resulting from the migration of Ni from spinel phase to the catalyst surface. Zhan et al. [31] focused on the tuning of the Ni particle size in Ni/Mg (Al)O via reduction temperature in order to decrease the amount of deposited coke during CH₄–CO₂ reforming. The highest activity was obtained on the reduced catalyst at 700 °C, while the reduction temperature of 750 and 800 °C led to a lower activity due to the sintering and severe coke deposition. They reported a critical size of about 6 nm for Ni to suppress coke formation.

SiO₂ has demonstrated to expose high surface area exposing more accessible active sites for adsorption and reaction [32]. Moreover, SiO₂ is introduced as one of the promising support for nickel based catalyst which is effective for methane decomposition [33]. Nonetheless, the low dispersion capability of SiO₂ due to weak interaction with active phase leads to acceleration of sintering and active phase particle agglomerations [34]. Hence, it appeared that, modification of the SiO2 supported catalysts by MgO with high thermal stability [35] might have led to improved catalytic activity and stability. Moreover, one knows that, the MgO species is used widely due to its strong basic sites, leading to large extents of CO₂ and H₂O production, which in turn prevent coke deposition due to progress in reverse CO disproportionation and carbon gasification reactions [[36], [37], [38]]. Furthermore, combination of the NiO and MgO species would cause solid solution formation during calcination process. The isolation of the Ni²⁺ by Mg²⁺ prevented migration of the Ni species upon the catalyst surface hence, sintering. In addition, strong metal-support interaction in solid solution structure created highly dispersed small size Ni species, improving the resistance of catalyst against coke deposition [39,40].

NiO–MgO–SiO₂ catalysts were synthesized through co-precipitation method, which further were subjected to hydrothermal treatment. The preparation method parameters were selected based upon recently published research [41], which led to very desirable physicochemical properties of final samples. Ultimately, the influence of the Ni content, reduction temperature, feed’s gas hourly space velocity (GHSV), and steam to carbon (S/C) ratio upon the catalytic performance and coke-deposition were understudied for the propane steam reforming towards producing syngas.