Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts

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Abstract

CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts prepared with various Ni loadings. The preparation of alumina supported Ni catalysts via sol–gel synthesis and subsequent supercritical drying led to the formation of very small metal particles, which are evenly distributed over the alumina support. The activity of the aerogel catalysts increased along with increasing metal loading, and eventually, the SAA25 (0.25 in Ni/Al mole ratio) catalyst exhibited the high activity comparable to that of a 5 wt.% Ru/alumina catalyst (ESCAT44, Engelhard). Compared to the alumina-supported Ni catalyst prepared by conventional impregnation method, Ni–alumina aerogel catalysts showed a remarkably low coking rate due to highly dispersed metal particles. From TEM micrograph studies, it was observed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. The loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering.

Introduction

During the past decades, CO2 reforming of CH4 has attracted interest from both industrial and environmental perspectives. In the environmental aspect, both CO2 and CH4 are recognized as undesirable greenhouse gases, and hence, the reaction provides a method of consuming CO2 effectively. From the industrial viewpoint, conversion of CH4 and CO2 into useful products is an important area of current catalytic research, especially C1 chemistry, because these two gases are the cheapest and most abundant carbon-containing materials and are convertible to synthesis gas with H2/CO ratio lower than that of steam reforming [1], [2].

The CO2 reforming reaction has been studied over numerous supported metal catalysts including Ni-based catalysts as well as noble metal-based ones [3], [4], [5], [6], [7], [8]. The latter have been reported to be more active and less sensitive to coking than the former. However, considering the aspects of high cost and limited availability of noble metals, it is more practical, from the industrial standpoint, to develop Ni-based catalysts which are resistant to carbon deposition, while exhibiting high activity for this reaction [4], [9].

Recently, we developed a method to prepare alumina aerogels having specific surface areas in excess of 700 m2/g and high thermal stability and successfully applied this method to prepare stable Ni–alumina aerogel catalysts for CH4 reforming with CO2 [10], [11]. The result of catalytic tests showed that these catalysts were suitable for the CO2 reforming reaction due to good activity and remarkable resistance to coke formation [11].

In this study, we tried to find the optimum level of metal loading which could provide Ni–alumina aerogel catalysts with high activity and coke resistance, by testing and characterizing the aerogel catalysts prepared with various metal loading amounts. Especially, our work has been mainly focused on elucidating the causes of deactivation and offering some criteria of filamentous carbon formation during the reforming reaction, in relation to the morphology of metal particles. In some studies published recently, much effort has been devoted to clarify the relationship between the carbon-forming behavior of supported Ni catalysts and the morphology of metal particles [12], [13], [14], [15], [16]. According to these studies, the carbon-forming tendency of supported Ni catalysts may have a close relationship with the metal particle size. However, unfortunately, the supported Ni catalysts prepared by conventional impregnation method could not offer a clear explanation about the effect of metal particle size on coke formation because the morphological control of metal particles was quite limited. On the other hand, the metal particle size could be properly controlled by varying Ni loading in the preparation step of sol–gel process and subsequent supercritical drying and thermal treatment.

In this work, we showed the presence of a minimum particle size to initiate the growth of filamentous carbon and we found that the sintering of metal particles was another major factor to deactivate supported Ni catalysts.

Section snippets

Catalyst preparation

Ni–alumina aerogel catalysts were prepared by the sol–gel processing of nickel acetate and aluminum sec-butoxide (ASB) in ethanol and subsequent supercritical drying with CO2 at 333 K and 24 MPa (designated as SAAx, where x means the Ni/Al mole ratio multiplied by 100). The dried aerogel was subjected to our standard calcination procedure, which consisted of heating in helium at 573 K and in oxygen at 773 K [17]. Further details of the procedure have been described previously [10], [11]. For

Properties of Ni–alumina aerogel catalysts

The textural properties of Ni–alumina aerogel catalysts (SAA series) are compared in Table 1 with those of 10 wt.% Ni supported on alumina catalyst (ICN, 0.096 in Ni/Al mole ratio) prepared by conventional impregnation method. Aerogel-based catalysts via sol–gel synthesis and subsequent supercritical drying exhibited high surface area, large pore volume, and pronounced mesoporosity. These catalytically favorable textural properties were well preserved even when the materials were treated at high

Conclusions

The present study demonstrates that the high-surface area Ni–alumina aerogel catalysts with mesoporosity could be used as suitable catalysts for CO2 reforming of CH4. Good textural properties and stability during the thermal treatment up to 973 K led to the formation of small Ni particles dispersed evenly over alumina support. By varying the Ni loading, the control of metal particle size can be achieved.

The reaction tests showed that the activities became higher with the increase in Ni loading,

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