Ethanol steam reforming on Mg- and Ca-modified Cu–Ni/SBA-15 catalysts

Abstract

The effect of Mg and Ca incorporation (0–20 wt%) into CuNi/SBA-15 catalysts for hydrogen production by ethanol steam reforming has been studied. Promoting elements, Ca and Mg, were added to SBA-15 support prior to the active phase, Cu (2 wt%) and Ni (7 wt%). In both cases, the metals were incorporated to SBA-15 by incipient wetness impregnation followed by calcination to obtain the corresponding oxides. XRD analyses and TEM images demonstrated that CaO and MgO improved the dispersion of the Cu–Ni phase. Moreover, TPR profiles showed that Ca or Mg strengthened the interaction between the SBA-15 support and the Cu–Ni phase. Both promoting effects of Ca and Mg, together with their basic character enhanced the catalytic performance of CuNi/SBA-15 catalysts on ethanol steam reforming, giving higher hydrogen selectivity and lower coke deposition.

Introduction

Owing to the global economic growth, population increase and technological development, the world primary energy demand, currently dominated by fossil fuels in more than 80%, is projected to expand by 53% from 2004 to 2030, reaching 17.1 billion tonnes of oil equivalent [1]. According to the International Energy Agency, if the situation does not change, it is estimated that global carbon dioxide emissions will increase by 1.7% per year, reaching 40 Gt/year in 2030. This would result in critical environmental problems all over the world [2]. In accordance with the Intergovernmental Panel on Climate Change (IPCC) [3], to stabilize carbon dioxide concentration into atmosphere in 550 ppm, it is necessary to obtain a reduction in carbon dioxide emissions of 50–60% before 2050. The use of hydrogen as an energy vector is an interesting and promising challenge. Hydrogen can be renewably produced in several ways and its utilization to obtain energy generates just water and no pollutant emission.

In this sense, ethanol has demonstrated to have good properties for hydrogen production [4]. Moreover, ethanol brings about a series of advantages, mainly handling safety and a renewable origin, as it can be produced in high amounts from several biomass sources (sugar and starchy crops, agricultural residues, wood and municipal solid wastes) [5]. Besides, the use of ethanol may avoid inconveniences of fossil fuels reserves, such as their limitation and location in politically unstable areas. Therefore, among the several ways to obtain hydrogen, ethanol steam reforming has become an interesting alternative for hydrogen production. This process, represented in Eq. (1), consists of a complex network of reactions giving hydrogen and several by-products as ethylene, acetaldehyde, methane, carbon oxides, coke, etc. Among undesirable products, it is necessary to stand out carbon monoxide and coke, as they cause poisoning of fuel cells and reforming catalysts.

C₂H₅OH + 3H₂O → 2CO₂ + H₂ (ΔH°₂₉₈ = + 347.4 kJ/mol)

The use of an appropriate catalyst may favour reaction pathways that minimize the formation of undesirable compounds, enhancing selectivity towards main products [4]. Regarding the active phase, supported Co, Rh or Ni catalysts are the most commonly used. Nickel catalysts possess high activity and low cost as advantages [4], but they produce high amounts of deposited coke. According to several studies, coke formation on Ni catalysts is related to the formation of large ensembles of metal atoms [6], but small additions of other metals, such as Au [7] or Cu [8], enhance Ni dispersion and reduce carbon deposition. Concretely, Cu–Ni supported catalysts have shown good catalytic properties for ethanol steam reforming [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], but coke deposition is still the major drawback for Cu–Ni catalysts.

Apart from the catalyst active phase, the support plays an important role, on metal dispersion, distribution and also on the reaction pathway. In this sense, it is well known that acid supports favour ethanol dehydration to ethylene, which easily may be transformed into coke [6], [8], [19]. According to the above-mentioned comments, there are some strategies to improve the catalyst performance and reduce the amount of carbon deposited:

• Reducing the metallic phase particles size, which means both higher active surface area and preferential elimination of large Cu–Ni ensembles related to coke deposition and carbon nanofibres growth [20].

• Increasing the support basic properties in order to hinder reactions catalyzed by acid sites such as ethanol dehydration to ethylene and so decreases carbon formation [21], [22].

The first strategy may be accomplished by the use of supports with appropriate textural properties. In this sense, mesostructured materials have shown good characteristics for accommodating metallic particles [23], [24] due to their controllable pore size, pore volume, and high surface area. In previous works [8], [9], we have demonstrated the better performance of siliceous SBA-15 material, compared to silica, MCM-41, γ-alumina and ZSM-5 zeolite as support of Cu–Ni catalysts.

On the other hand, some authors [10], [11], [12], [21], [22], [25], [26], [27] have improved catalysts performance on reforming processes by adding alkaline elements, such as K, Mg or Ca in order to reduce catalysts acid sites. Most studies have been done with alumina supported Ni catalysts, where the addition of alkaline-earth elements improved catalyst stability. This was attributed to the reduction of coke formation or even to a retarded Ni sintering by improving the Ni dispersion and strengthening the nickel–alumina interaction [22], [27]. The influence of alkaline-earth elements on the activity of supported Ni catalysts depends on the amount added and the properties of the support [22]. Moreover, Cheng et al. [25] described that the promoting effect is more significant when the support is impregnated with the promoter before the incorporation of the active phase.

For these reasons, it has been considered interesting to prepare Cu–Ni catalysts supported on Mg- and Ca-modified SBA-15 mesoporous materials and to test them in the hydrogen production through ethanol steam reforming.