Background
Due to environmental concerns, there is an increasing interest in technologies that offer an alternative to fossil-based processes. An example is the Fischer-Tropsch synthesis (FTS), which offers a route to convert e.g. municipal waste, biomass, and ultimately CO₂ via CO/H₂ mixtures into fuels and chemicals. FTS is commercially conducted with cobalt- and iron-based solid catalysts. Linear hydrocarbons are the targeted output of the cobalt-based FTS. Besides that, hydrocarbon and carbonaceous species will be formed during the FTS and are considered a cause of catalyst deactivation when irreversibly adsorbed on the catalyst surface. Early-stage detection of these species has been a challenge in the field from an analytical and experimental perspective.
Methods
Herein, we demonstrated the potential of nanoscale infrared spectroscopy (nano IR) to analyze early-stage formation of hydrocarbon and carbonaceous species (see Fig. 1). Nano IR is a hybrid analytical technique, which combines the spatial resolution of atomic force microscopy (AFM) with the chemical information from infrared (IR) spectroscopy. The complexity and roughness of heterogeneous catalyst materials posed experimental challenges for analysis with nano IR. To overcome this, we have developed a Co/TiO₂ micro-island model catalyst.
Results and Conclusions
We demonstrated the existence of polyaromatic hydrocarbons on the Co/TiO₂ catalyst surface and their emergence via aldehyde functionalities. These findings have direct ramifications for the formation mechanism. Although not unquestioned, conventional (linear) FTS products are believed to form via direct dissociation of CO, followed by hydrogenation of C and polymerization of CH_x. Our research provides evidence that polyaromatic hydrocarbons are formed via a different mechanism, namely H-assisted C-O bond scission (see Fig. 1).

<b>Fig. </b><b>1</b><b>. </b><b>| </b>Methodology to identify (hydro)carbons on Co/TiO<sub>2</sub> (left). The two main mechanisms in FTS (middle): Polyaromatics form <i>via</i> H-assisted C-O bond scission (middle left) and linear hydrocarbons <i>via </i>direct dissociation of CO (middle right). Nano IR spectra at 0, 1, 2, 6, 24 h FTS reaction (right).

Fig. 1. | Methodology to identify (hydro)carbons on Co/TiO₂ (left). The two main mechanisms in FTS (middle): Polyaromatics form via H-assisted C-O bond scission (middle left) and linear hydrocarbons via direct dissociation of CO (middle right). Nano IR spectra at 0, 1, 2, 6, 24 h FTS reaction (right).

Stephan Kupfer

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