(159s) Rate and Selectivity Oscillations during the Fischer-Tropsch Reaction over Ceria Supported Co/Cu Catalysts

Co-based catalysts stand out for their capability of forming chain-lengthened oxygenates with high selectivities in the Fischer-Tropsch Synthesis (FTS), provided reducible metal oxides are used as support. Our group has previously reported an activity–selectivity hysteresis for Co/MnO_x catalysts during FTS and attributed this phenomenon to a reaction-induced Co–Co₂C phase transition¹. Compared with metallic Co, where CO molecules may undergo dissociative chemisorption, Co₂C is largely inert in this regard. The research presented here will demonstrate, for the first time, rate and selectivity oscillations in the syngas conversion over a Co-based catalyst using ceria as a support.

Materials and Methods

Co₁Cu₁Ce₁ and Co₁Cu₁ catalysts (indices indicating atomic ratios) were prepared via the oxalate route using metal nitrate precursors and oxalic acid as precipitation agent in acetone. Co₄Mn₁K_0.1 was prepared according to the literature¹.

Experiments were performed in a home-built experimental setup allowing studies of Chemical Transient Kinetics (CTK) as well as Temperature-Programmed Desorption/Decomposition (TPD/TPDec) and BET/H₂-D₂ exchange without sample displacement. Data analysis was performed with an online quadrupole mass spectrometer (or GCMS, if necessary). The fixed-bed flow reactor was operated at close-to-CSTR conditions, as previously described². This is an essential feature of the methodological approach toward quantification of volumetric molecular flows. Calcined oxalates were subjected to an in-situ decomposition in H₂ at 450°C for 1h. While Co and Cu mixed oxalates largely decomposed to the metallic state, Ce- Mn- and K-oxalates remained in an oxidized state. Activated catalysts were cooled to reaction temperature in a mixture of H₂/He. After equilibration, the flow of non-reactive H₂/He was abruptly replaced by a flow of reactive CO/H₂/Ar while keeping the total gas flow rate and the H₂ partial pressure constant. Shortly after switching to “build-up” conditions, reaction rate and product selectivities started oscillating. Depending on catalyst composition and experimental conditions, self-sustained oscillations were observed for hours of time-on-stream.

Results and Discussion

Figure 1 (a) shows oscillatory behavior of reactants and products in the catalytic CO hydrogenation over an activated Co₁Cu₁Ce₁ catalyst. For the sake of clarity, the vertical scaling has been arbitrarily adapted for each component. Fourier transform analysis provides an oscillation frequency of 0.00411 Hz (period=243 s) for reactants and products. The observed oscillatory reaction behavior is non-isothermal. This is evidenced by periodic changes in the temperature with the same frequency. On the one hand, since FTS is a strongly exothermic reaction, local variations of the temperature are to be expected. On the other hand, the occurrence of uniform oscillations with well-defined frequency and amplitude (~4 to 9 °C) over hours of time-on-stream can only be explained by assuming efficient thermal coupling between catalyst particles leading to a high degree of synchronization of rate-and-selectivity oscillations, as seen in our case. Reactant and product formation are out-of-phase, i.e. a phase shift of Ï€ relative to the reactants, is observed. This is most clearly demonstrated for CO₂, CH₄, and C₂H₄ in Figure 1 (b). Other hydrocarbons such as C₂H₆, C₃H₆, and C₄H₁₀ cannot be easily evaluated due to relatively low signal-to-noise ratios. We also refrain from evaluating the behavior of water since it is subject to a “chromatographic effect” in the capillary leading to the mass spectrometer. On the other hand, C₃H₈ systematically shows up with a phase shift shorter than Ï€; more data is necessary to elaborate on this effect.

Oscillation periods as long as those observed in the present study have not been reported in the literature. To explain self-sustained oscillatory behavior with uniform periods of 243 s, a particularly slow feedback mechanism must be involved to drive the oscillations. We hypothesize that a reversible reaction-induced transformation of the catalytically active phase is in operation. In particular, we envisage the Co-to-Co₂C conversion to play a dominant role in this scenario. On the other hand, it also seems clear that catalysts in the absence of ceria support only show transient oscillations (a few periods), if any, see Figure 1 (c).

Figure 1. (a) Oscillatory behavior in the CO/H₂ conversion and product formation over Co₁Cu₁Ce₁. Reaction conditions: T = 245°C, P = 1 atm, H₂/CO = 6/1, Total flow rate = 30 ml/min. For the sake of clarity, the vertical scaling has been arbitrarily adapted for each component. (b) Phases of the oscillation signals in (a). (c) Temperature profiles of CO hydrogenation over Co₁Cu₁ and Co₄Mn₁K_0.1 catalysts at different temperatures and H₂/CO ratios, at otherwise identical reaction conditions as in (a).

Implications

The occurrence of rate and selectivity oscillations in syngas conversion over a Co₁Cu₁Ce₁ catalyst has been demonstrated for the first time. While the reactants oscillate in-phase, most products, including CO₂, CH₄, and C₂H₄, apparently oscillate with a phase shift of Ï€ relative to the reactants. We hypothesize that the reversible reaction-induced Co-Co₂C phase transformation plays a role in the feedback mechanism for the oscillations.

References

1. Xiang, Y., Kovarik, L. & Kruse, N. Rate and selectivity hysteresis during the carbon monoxide hydrogenation over promoted Co/MnOx catalysts. Nat. Commun. 10, (2019).

2. Athariboroujeny, M. et al. Competing Mechanisms in CO Hydrogenation over Co-MnOx Catalysts_supporting. ACS Catal. 9, 5603–5612 (2019).