ToF-SIMS as a versatile tool to study the surface properties of silica supported cobalt catalyst for Fischer–Tropsch synthesis

Highlights

• Application of ToF-SIMS to the characterization of cobalt catalysts for FT synthesis.

• Direct confirmation of cobalt silicate formation on the Co/SiO₂ surface.

• New ToF-SIMS procedure allowed to determine an exact composition of Co catalyst.

Abstract

The aim of this work was to determine the surface properties of silica supported cobalt catalyst for Fischer–Tropsch synthesis. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used for this purpose as a main analytical tool. Moreover, X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis/differential thermal analysis/mass spectrometry (TG–DTA–MS) and BET surface area measurements were performed. The influence of a preparation method and treatment conditions on the surface composition (including the formation of different phases of cobalt oxides, cobalt silicate and decomposition of cobalt nitrate – catalyst precursor), cobalt oxidation degree and metal dispersion was studied. 10%Co/SiO₂ catalysts prepared by impregnation and deposition–precipitation methods were investigated. Moreover, pure Co₃O₄, CoO and Co powders were used as reference materials.

Introduction

One of the large scale processes in petrochemical industry is Fischer–Tropsch synthesis (FTS), which seems to be the most promising way for the production of synthetic fuels from coal, natural gas or biomass [1], [2], [3]. Owing to that, a final product with a high cetane number, and a low content of sulfur and aromatics, can be obtained, which decreases its impact on the environment and makes it possible to be applied in diesel engines.

Supported cobalt is one of the most popular catalysts for Fischer–Tropsch synthesis [4], [5], [6]. Literature data reveal that cobalt based catalysts are relatively inexpensive and have a high selectivity for heavy hydrocarbons, and a low water–gas shift reaction activity [7]. Their catalytic properties depend on the number of active sites on the catalyst surface after the reduction process, which can be determined by metallic Co particle size, loading amount, reduction degree and chemical/texture properties of the support [8]. It appears that the preparation of highly dispersed supported cobalt catalysts requires the formation of a strong interaction between the support and the Co precursor [9]. On the other hand, the strong interaction mentioned above diminishes the reducibility of supported cobalt species. Therefore, due to the formation of cobalt silicate, an increase in the reduction temperature of the prepared material is necessary. However, high reduction temperature may favor the sintering of cobalt crystallites. It means that the preparation of highly active and selective catalyst demands a compromise between the cobalt dispersion and its reduction degree [10]. It was demonstrated that due to its high surface area, porosity and stability, one of the most favorable supports used in the formation of such systems is SiO₂ [11]. Nevertheless, also in this case the choice of an appropriate preparation method and suitable treatment conditions is critical for the catalytic properties of the prepared material. Knowledge about changes in the surface composition which occur during subsequent steps of the catalyst preparation is necessary for this purpose. It can be obtained by the use of various analytical methods [12], [13], [14], [15], [16]. In spite of a great potential of the applied techniques, each of them have also considerable limitations. For instance, in X-ray diffraction (XRD) experiment only the presence of crystalline phase can be observed. In the case of scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) measurements, besides structural information, only elemental composition of the studied sample can be determined. Furthermore, the distinction of cobalt precursor (i.e. cobalt nitrate) and cobalt silicate formed during the sample treatment is very difficult on the basis of X-ray photoelectron spectroscopy (XPS) spectra of a Co catalyst [17]. In addition in the case of XPS experiments the information origins from the depth of 1–10 nm of the investigated material while for ToF-SIMS it is possible to obtain data from the top 1 nm of the surface layer.

These facts indicate the need of the use of several complementary methods in order to gain complete information about the studied surface. Moreover, the limitations mentioned above stimulate the search for new techniques, which would allow to obtain additional data of the investigated catalysts. Therefore, we propose an application of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to the studies of such a type of materials. The main advantages of ToF-SIMS are associated with the relatively high sensitivity of secondary ion mass spectrometers, the possibility of detection of not only elemental, but also molecular ions, and the fact that ToF-SIMS can give information from the upper layer of the investigated material. On the other hand, difficulties in the interpretation of mass spectra are generally related to the matrix effect, fragmentation reactions and quantification problems [18], [19], [20]. Especially the matrix effect may hinder the comparison of the intensity of the signals observed on the mass spectra. Therefore, the results should be normalized before their interpretation, but even then usually only semi-quantitative analysis is possible. An application of ToF-SIMS to the studies of heterogeneous catalysts gives a possibility of determination of their surface composition, characterization of interactions between the active phase and the support, estimation of the active phase dispersion on the analyzed surface, comparison of the metal oxidation degree, study of catalyst deactivation processes and determination of the presence of the catalyst precursors and investigations of their decomposition [21], [22], [23], [24], [25], [26], [27]. These capabilities indicate that ToF-SIMS can be a very useful tool in the analysis of a Co/SiO₂ catalyst, and owing to the use of this technique, supplementary information of the studied surface can be obtained [28], [29].

This work was focused on the determination of the surface properties of silica supported cobalt catalyst for Fischer–Tropsch synthesis. Both the influence of the preparation method and treatment conditions on the composition of the surface of the studied catalysts have been investigated. An application of ToF-SIMS allowed to not only detect the presence of cobalt silicate formation, but also to determine particular phases of cobalt oxides on the catalyst surface. Especially, the latter was difficult using this technique due to fragmentation and recombination reactions which proceed during the measurement. In that case the ions characteristic of cobalt and cobalt oxides (i.e. ${\text{Co}}^{+}\text{,}{\text{CoO}}^{-}\text{,}{\text{CoO}}_2^{-}\text{,}{\text{CoO}}_3^{-}$) are present on the all mass spectra collected for the samples containing just Co, CoO or Co₃O₄ phase. In order to overcome this problem we propose a new procedure, which allows to obtain precise information about the phase composition of the Co/SiO₂ catalyst.