Fischer−Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts

Abstract

The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hägg carbide and Fe3O4. After 10 h of CO/He treatment at 290 °C, the main component observed was Hägg carbide. A model containing Hägg and ε-carbides, and Fe3O4, was also investigated. However, the r-factor was not significantly impacted by including ε-carbide in the fitting, and the resulting contribution of ε-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.

Fischer–Tropsch Synthesis: Deactivation as a Function of Potassium Promoter Loading for Precipitated Iron Catalyst

“…These peaks are associated with reduction of Fe 2 O 3 to lower oxides (i.e., Fe 3 O 4 and defect-laden form of this oxide) prior to carburization. The peaks are fully consistent with our previous CO-TPR XANES/EXAFS studies [33][34][35]. The second peak is located in the temperature range of 350-525°C.…”

Ruthenium particle size and cesium promotion effects in Fischer–Tropsch synthesis over high-surface-area graphite supported catalysts

“…In this sense, the alkali promoter shifts the hydrocarbon selectivity towards longer chain hydrocarbons and olefin products. 6 Furthermore, it has been reported that such alkali elements (Na, K or Cs) are electronic promoters that transfer part of their electron density to Ru even in its oxidized state, which strengthens the CO-metal interaction whereas weakening the C-O bond. 4,[7][8][9] Recently, some experimental evidences of the partial reduction of Cs in Cs-Ru/C catalysts during temperatureprogrammed reduction have been provided by X-ray absorption spectroscopy.…”

The role of potassium promoter in surface carbon hydrogenation on Hägg carbide surfaces

“…Promotion effect of alkali metals has attracted continuous interests for several decades in heterogeneous catalysis [1][2][3], such as in Fischer-Tropsch synthesis (FTS) [4,5], ammonia synthesis [6], steam reforming of methane [7], low alcohol synthesis [8] and epoxidation reaction [9]. Although many studies focused on promotion effect of alkali metals on metal surfaces [10][11][12], the mechanisms in affecting the performance of catalysts still remain controversially.…”

“…It is claimed that potassium promoter can enhance the activity of FTS [5] and water-gas shift reaction (WGS) [14], and increase the selectivity of the desired heavy hydrocarbons [15]. In addition, potassium promoter is known to promote CO adsorption and dissociation [16][17][18], improve (or suppress) H 2 adsorption [19][20][21], increase the carburization rate of iron catalysts [3], and suppress CH 4 formation [22]. Recently, K 2 O promotion has been proposed to modify the Fe crystallite morphology in stabilizing the more active facets [23].…”