Effects of metal loading and support for supported cobalt catalyst
Graphical abstract
Highlights
► Cobalt-support interaction depends on specific support and loading. ► In situ DRIFTS during CO2 methanation reveals adsorbed CO and formate species. ► Reactivity of supported cobalt catalysts depends on metal loading and support. ► Simultaneous reactivity measurements reveal high cobalt loadings of ceria supported catalysts as the best.
Introduction
Supported cobalt catalysts are used for various chemical reactions such as Fischer–Tropsch synthesis (FTS), CO2 hydrogenation, steam reforming of ethanol and methane, hydrogen production, CO preferential oxidation, soot conversion, and hydrodesulfurization [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Various supports have been used to prepare the cobalt based catalysts, such as SiO2, Al2O3, MgO, TiO2, Nb2O5, CeO2, and ZrO2 [6], [11], [12], [13], [14], [15]. Previous studies reveal that during preparation of some of these supported cobalt catalysts, cobalt-aluminate, cobalt-silicate, cobalt-niobate, cobalt-titanate, and cobalt-magnesia solid solutions were formed. The formation of these cobalt-support compounds may be detrimental to the catalytic activity of the cobalt metal since they are hard to reduce [1], [7], [8], [12], [13], [14], [15], [16], [17]. Consequently, the formation of cobalt metal during reduction depends on the nature and interaction of specific support with the cobalt metal. Furthermore, these interactions of cobalt with the support may also depend on the metal loading.
Supported cobalt catalysts have been studied by various characterization techniques. Some of the techniques considered to study supported cobalt catalysts are: Raman spectroscopy [1], Fourier transform infrared (FTIR) spectroscopy [2], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], XRD [1], [10], [20], [23], [27], [28], UV–vis–NIR spectroscopy [10], [12], [27], [28], [29], [30], and X-ray photoelectron spectroscopy (XPS) [1], [7], [8], [12], [13], [14], [15], [16], [17], [23].
In the present study several series of supported cobalt catalysts were synthesized to observe the effect of support and loading. The supports considered were: silica (SiO2), alumina (Al2O3), magnesia (MgO), titania (TiO2), niobia (Nb2O5), ceria (CeO2), and zirconia (ZrO2). The supports and supported catalysts were characterized by XRD, UV–vis–NIR, and DRIFT spectroscopy obtained during the CO2 hydrogenation reaction. The simultaneous collection of conversion and yield data with the corresponding in situ IR spectra were obtained. The surface areas of the supports and supported catalysts were also determined. Based on the combined information from the above characterization and reactivity studies, the effect of the support and loading was proposed.
Section snippets
Material synthesis
Several series of cobalt catalysts containing 5–20 wt % cobalt on various supports were synthesized by the incipient wetness impregnation method. The cobalt precursor was cobalt (II) nitrate hexa hydrate (Merck, 98%) and the supports were silica (SiO2), alumina (Al2O3), magnesia (MgO), titania (TiO2), niobia (Nb2O5), zirconia (ZrO2), and ceria (CeO2). Of these supports, MgO and ZrO2 were synthesized using available precursors and the remaining supports were obtained from commercially sources.
Surface area analysis
The surface areas of the supports and supported catalysts were measured and changes in each series were analyzed. The alumina and silica supports had a surface of about 190 m2/g, the titania and ceria supports had a surface area of about 50 m2/g, the niobia and zirconia supports had a surface area of about 38 m2/g and the magnesia support had a surface area of 18 m2/g. The surface area of the supported catalysts were normalized with the surface area of the support and presented together in Fig. 1,
Conclusion
Series of silica, alumina, magnesia, titania, niobia, zirconia, and ceria supported cobalt catalysts were synthesized, characterized and tested for the CO2 hydrogenation reaction to understand the effect of loading and support. The surface area of the catalysts decreased with increasing cobalt loading, except for the magnesia supported cobalt catalyst. For magnesia supported cobalt catalysts, however, the surface area significantly increases. Characterization of the catalysts revealed that the
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