Metal–support interaction effects on the growth of filamentous carbon over Co/SiO2 catalysts
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.
Section snippets
Catalyst preparation
The Co/SiO2 catalyst was prepared by incipient wetness impregnation following the procedures described in previous studies [21], [22], [23]. The Co/BaO/SiO2, Co/La2O3/SiO2 and Co/ZrO2/SiO2, catalysts were prepared by step-wise incipient wetness impregnation. The pre-calcined (773 K for 25 h in air) silica support (silica gel, grade 62, 60–200 mesh, 15A, Aldrich 24398-1) was impregnated with an aqueous solution of Ba(NO3)2 (>99.1%) or La(NO3)3·6H2O (99.9% REO) or ZrOCl2·8H2O (99.9% metals basis).
Catalyst reduction
To determine the effect of BaO, ZrO2, and La2O3 addition on the reduction behavior of Co3O4, catalyst samples were characterized by TPR. The TPR profiles of the Co/SiO2, Co/BaO/SiO2, Co/ZrO2/SiO2 and Co/La2O3/SiO2 catalysts are shown in Fig. 1. Generally, the TPR profiles could be resolved into three peaks. The peak position and relative intensity, representing the relative H2 consumption of each peak, are summarized in Table 1.
The TPR profile of the Co/SiO2 catalyst of Fig. 1a shows two main
MSI effects
MSI effects are known to influence the reduction of metal oxide precursors and metal dispersion on supported metal catalysts [27], [30], [32], [34], [35], [36]. A strong MSI increases the difficulty of the reduction of the precursor oxide, either by increasing the reduction temperature of existing oxides or by the production of metal–support species (such as Co2SiO4) that are difficult to reduce or are irreducible. Furthermore, a strong MSI decreases the mobility of the metal surface species
Conclusions
The effect of BaO, La2O3 and ZrO2, added to the SiO2 support of Co catalysts, has been investigated. The effect of the modified support on the catalyst reduction behavior, dispersion and MSI was studied by TPR, XPS and CO chemisorption. The results suggest an increasing MSI among the catalysts in the order Co/SiO2≈Co/BaO/SiO2<Co/La2O3/SiO2<Co/ZrO2/SiO2. The rate of catalyst deactivation was affected by the modified support: increased deactivation corresponded to an increased MSI. It is
Acknowledgements
Funding for the present study from the Natural Science and Engineering Research Council of Canada is gratefully acknowledged. We gratefully acknowledge Ken Wong, Phil Wong and Dr. Keith A.R. Mitchell for XPS measurements and Dr. Chang Chun Yu for helpful discussions of the XPS results. Li Xiao-Nian expresses thanks to the Pao Yu-Kong and Pao Zhao-Long Scholarship Foundation for financial support.
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