Metal–support interaction effects on the growth of filamentous carbon over Co/SiO₂ catalysts

Abstract

The addition of BaO, La₂O₃ and ZrO₂ to the SiO₂ support of a 12 wt.% Co/SiO₂ catalyst modifies the reduction behavior of Co species and leads to changes in metal dispersion. These changes are due to a modified metal–support interaction (MSI) between cobalt species and the Me_xO_y/SiO₂ (Me_xO_y: BaO, ZrO₂ and La₂O₃) support. Catalyst characterization by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) have been used to determine the relative strength of the MSI and the results suggest an increasing MSI in the order Co/SiO₂≈Co/BaO/SiO₂<Co/La₂O₃/SiO₂<Co/ZrO₂/SiO₂. The rate of catalyst deactivation during methane decomposition (CH₄⇔C+2H₂) is shown to increase with increasing MSI. Analysis of the used catalysts also shows that an increasing rate of deactivation correlates with an increasing amount of graphitic carbon versus carbidic carbon on the used catalyst. It is suggested that an increase in graphitic carbon is a consequence of a strong MSI that limits carbon removal from the metal surface by filament formation. Consequently, graphitic, encapsulating carbon is formed from the carbon deposited during methane decomposition, leading to deactivation of the catalyst.

Introduction

Methane conversion to more valuable products is of interest because of the existence of large reserves of natural gas (>80% CH₄ by volume), petroleum-associated gas, and methane hydrate [1], [2]. Direct methane conversion by methane oxidative coupling, methane aromatisation and methane homologation, or indirect methane conversion such as conventional steam reforming, dry reforming and partial oxidation, have all been described in the literature [3], [4], [5], [6], [7]. More recently a cyclic process in which methane cracking is followed by gasification of carbon by steam or oxygen, in order to produce high purity hydrogen and new carbon materials, has also been investigated [8], [9].

In all of these CH₄ upgrading processes, the activity of the catalyst for CH₄ activation and the carbon species formed during CH₄ decomposition are important, since both influence the life of the catalyst and the selectivity and yield to the desired products [5], [6], [11], [12]. Catalysts studied for CH₄ decomposition include various metals (Pd, Pt, Rh, Ru, Ir, Re, Co, Ni, W, Fe, Mn, Mo, Ta, Cu, Ag, Cr, Ti, V₂O₅, and Mo), dispersed on supports (Al₂O₃, SiO₂, MgO, TiO₂, Al₂O₃.SiO₂, La₂O₃, ZrO₂, and HZSM-5, NaY zeolite), and promoted using a range of elements (K, Cu, Mg, Al, Ca, V, Mn, Fe, Ga, Co, Zn, Sr, Mo, W, Ru, Rh, Pd, Ag, and Pt ) [10], [12], [13], [14], [15], [16]. The best catalysts are Group VIII transition metals (Pd, Rh, Ru, Ni, Co or Fe) supported on silica, alumina, or zeolite, depending on the experimental conditions [10], [12], [13], [17], [18], [19]. The present study is focused on Co-based catalysts that have high chain growth probability in Fischer–Tropsch synthesis [20] and high selectivity for C₂₊ hydrocarbons in the homologation of methane [21].

Previous studies of CH₄ decomposition on Co/SiO₂ catalysts at moderate temperatures showed that carbon nanofibers were formed at specific reaction conditions and that the conditions required were similar to those required with Ni catalysts [22], [23]. The mechanism for carbon nanofiber formation described in the literature [25] assumes that CH₄ decomposes on the catalyst metal surface according to the overall reaction CH₄⇔C+2H₂, forming single carbon atoms. The carbon atoms dissolve in the metal and diffuse through the metal particle, although some surface diffusion around the outside of the particle cannot be excluded [24]. The carbon precipitates in the form of graphite at the interface between the metal particle and the support. The metal particle is detached from the support by the formation of carbon layers. The carbon nanofiber continues to grow and the metal surface remains active since the carbon deposited by CH₄ decomposition is removed from the surface by diffusion through the particle. The consequence of this mechanism is that a stable activity for CH₄ decomposition is observed. According to the mechanism, the interaction between the catalyst metal particle and the support will play a role during the growth of the nanofiber. Snoeck et al. [25] noted that a strong metal–support interaction will not only affect the deformation of the metal particle at the tip of the nanofibers, but will also influence the nanofiber morphology, yielding hollow or solid fibers.

Co catalysts exhibit a metal–support interaction (MSI) that influences the Co dispersion and the reduction of cobalt ions on the support [27], [28], [30], [32], [34], [35], [36]. The effects of promoters such as a second transition metal (Ru) [26], alkali metal (K) [16], rare earth (La₂O₃) [27], and/or a transition metal oxide (ZrO₂) [28], on Co dispersion and reduction of cobalt ions on the support, have also been described in the literature. In the present study, the MSI exhibited by Co/SiO₂ catalysts have been modified by the addition of BaO, La₂O₃ and ZrO₂ to the SiO₂ support prior to the dispersion of Co. The influence of the modified support on the MSI and hence the reduction of Co species and the Co dispersion has been studied by TPR, XPS and CO chemisorption. The influence of the MSI on the activity and stability of the Co catalysts during CH₄ decomposition, is also reported.