Electrochemical Promotion by Potassium of Rhodium-Catalyzed Fischer−Tropsch Synthesis:  XP Spectroscopy and Reaction Studies

Abstract

The catalytic behavior, chemical state, and morphology of a Rh thin film catalyst electrochemically promoted by alkali have been investigated by reactor studies, XPS, and SEM. At 1 bar pressure and 473 K, electropumped alkali decreases overall activity toward the production of hydrocarbons from CO and H 2 while increasing selectivity toward higher hydrocarbons, especially alkenes. On the active surface, rhodium is in a partially oxidized state; however, the catalytic activity is assigned to the metallic (Rh 0 ) component. The promoter phase is an alkali compound other than carbonate, although at high alkali loadings large amounts of crystalline alkali carbonate are deposited on the surface. The heavily carbonated system nevertheless retains most of its catalytic activity, with continued increase in selectivity toward higher hydrocarbons as the alkali loading is increased. Fully reversible behavior is found in every respect, including recovery of the initial state by electrochemical decomposition of the crystalline alkali carbonate deposit. The reaction mechanism is discussed with respect to the identity of the active site and the role of the alkali promoter.

Electrochemical Promotion by Potassium of Rh-Catalysed Fischer–Tropsch Synthesis at High Pressure

“…At higher pressures (5-14 bar) the maximum chain length increased to 12 and at 14 bar the largest number of products and greatest hydrocarbon chain length were observed. We have previously demonstrated by means of XPS that decreasing (increasing) V WR results in a reversible increase (decrease) in K surface coverage [12]. In agreement with this, under all the conditions explored in the present study, the catalytic response of the system was reproducible and reversible with respect to the catalyst potential: returning V WR to any given value restored the corresponding values of all reaction rates.…”

“…As to the actual K loadings present in these experiments, the relevant parameters are absolute amount and spatial distribution. Our earlier [12] post-reaction XPS data showed that maximum K loading corresponded to 50 monolayers of K having been supplied to the Rh surface. This result, combined with SEM, and EDX analyses [12] showed that most of the alkali was present as crystallites of (catalytically inert) potassium carbonate with sub-monolayer amounts of K promoting the catalytically active Rh surface.…”

“…Our earlier [12] post-reaction XPS data showed that maximum K loading corresponded to 50 monolayers of K having been supplied to the Rh surface. This result, combined with SEM, and EDX analyses [12] showed that most of the alkali was present as crystallites of (catalytically inert) potassium carbonate with sub-monolayer amounts of K promoting the catalytically active Rh surface. Figure 4a, b shows the steady-state rate and selectivity data for alcohol production as a function of catalyst potential.…”

“…We have already reported an EP study of Rh-catalysed FTS at atmospsheric pressure [12], in which the reactor was coupled to an XPS spectrometer chamber. This work demonstrated the applicability of EP to FTS, allowed us to identify surface compounds formed under reaction conditions, and uncovered certain aspects of the reaction mechanism.…”

A Review of Surface Analysis Techniques for the Investigation of the Phenomenon of Electrochemical Promotion of Catalysis with Alkaline Ionic Conductors

“…On the other hand, the first EPOC study using a K + -conductor electrolyte (K2YZr(PO4)3) dates from 1997 and addressed the Fe-catalyzed ammonia decomposition [20]. Urquhart et al used other K + -conductor solid electrolyte (K-βAl2O3) in Fischer-Tropsch reaction studies under both atmospheric [21] and high pressure [22], and de Lucas-Consuegra et al introduced the use of this kind of ion-conducting catalyst support for the electrochemical promotion of Pt in CO [23] and propylene [24] oxidation, as well as in NOx reduction reactions [25,26]. More recent alkaline electrochemical promotion studies on CO2 hydrogenation [27][28][29][30] and methanol conversion reactions [31][32][33] should Vayenas et al performed the first electrochemical promotion study with alkaline solid electrolyte (Na-βAl 2 O 3 ) in 1991 [8].…”