Robert Pestman

Structure sensitivity in the ruthenium nanoparticle catalyzed aqueous-phase Fischer-Tropsch reaction

Low-temperature Fischer–Tropsch reaction data are reported for Ru nanoparticles suspended in the water phase. Their activity and selectivity strongly depends on particle size, when varied between 1 to 5 nm. Small particles display high oxygenates selectivity. The Anderson–Schulz–Flory (ASF) chain-growth probability for oxygenates is significantly lower than that observed for hydrocarbons. The chain growth parameter for hydrocarbon formation is independent of particle size. For oxygenates it is constant only for particles larger than 3 nm. Oxygenate and hydrocarbon formation occur on different sites. The ASF chain-growth probability for oxygenate formation increases with temperature. For very small 1.2 nm particles it shows a maximum as a function of temperature. This unusual temperature dependence is due to relatively slow CO dissociation compared to the rate of C–C bond formation.

Effect of Organic Capping Agents on Ruthenium‐Nanoparticle‐Catalyzed Aqueous‐Phase Fischer–Tropsch Synthesis

The influence of organic capping agents on the performance of Ru nanoparticles in aqueous‐phase Fischer–Tropsch (FT) synthesis was investigated. Three organic capping agents were used: trimethyl(tetradecyl)ammonium bromide (TTAB), polyvinylpyrrolidone (PVP), and sodium 3‐mercapto‐1‐propanesulfonate (SMPS). To exclude the effects of particle size, the capping agents were placed onto carbon‐nanofiber‐supported Ru nanoparticles of size 3.4 nm. The activity in the FT reaction increased in the order: Ru‐SMPS≪Ru‐PVP

Mechanism of Cobalt-Catalyzed CO Hydrogenation:2. Fischer-Tropsch Synthesis

Fischer–Tropsch (FT) synthesis is one of the most complex catalyzed chemical reactions in which the chain-growth mechanism that leads to formation of long-chain hydrocarbons is not well understood yet. The present work provides deeper insight into the relation between the kinetics of the FT reaction on a silica-supported cobalt catalyst and the composition of the surface adsorbed layer. Cofeeding experiments of 12C3H6 with 13CO/H2 evidence that CHx surface intermediates are involved in chain growth and that chain growth is highly reversible. We present a model-based approach of steady-state isotopic transient kinetic analysis measurements at FT conditions involving hydrocarbon products containing up to five carbon atoms. Our data show that the rates of chain growth and chain decoupling are much higher than the rates of monomer formation and chain termination. An important corollary of the microkinetic model is that the fraction of free sites, which is mainly determined by CO pressure, has opposing effects on CO consumption rate and chain-growth probability. Lower CO pressure and more free sites leads to increased CO consumption rate but decreased chain-growth probability because of an increasing ratio of chain decoupling over chain growth. The preferred FT condition involves high CO pressure in which chain-growth probability is increased at the expense of the CO consumption rate.

Mechanism of carbon monoxide dissociation on a cobalt Fischer–Tropsch catalyst

The way in which the triple bond in CO dissociates, a key reaction step in the Fischer–Tropsch (FT) reaction, is a subject of intense debate. Direct CO dissociation on a Co catalyst was probed by 12C16O/13C18O scrambling in the absence and presence of H2. The initial scrambling rate without H2 was significantly higher than the rate of CO consumption under CO hydrogenation conditions, which indicated that the surface contained sites sufficiently reactive to dissociate CO without the assistance of H atoms. Only a small fraction of the surface was involved in CO scrambling. The minor influence of CO scrambling and CO residence time on the partial pressure of H2 showed that CO dissociation was not affected by the presence of H2. The positive H2 reaction order was correlated to the fact that the hydrogenation of adsorbed C and O atoms was slower than CO dissociation. Temperature‐programmed in situ IR spectroscopy underpinned the conclusion that CO dissociation does not require H atoms.

Mechanism of Cobalt-Catalyzed CO Hydrogenation: 1. Methanation

The mechanism of CO hydrogenation to CH4 at 260 °C on a cobalt catalyst is investigated using steady-state isotopic transient kinetic analysis (SSITKA) and backward and forward chemical transient kinetic analysis (CTKA). The dependence of CHx residence time is determined by 12CO/H2 → 13CO/H2 SSITKA as a function of the CO and H2 partial pressure and shows that the CH4 formation rate is mainly controlled by CHx hydrogenation rather than CO dissociation. Backward CO/H2 → H2 CTKA emphasizes the importance of H coverage on the slow CHx hydrogenation step. The H coverage strongly depends on the CO coverage, which is directly related to CO partial pressure. Combining SSITKA and backward CTKA allows determining that the amount of additional CH4 obtained during CTKA is nearly equal to the amount of CO adsorbed to the cobalt surface. Thus, under the given conditions overall barrier for CO hydrogenation to CH4 under methanation condition is lower than the CO adsorption energy. Forward CTKA measurements reveal that O hydrogenation to H2O is also a relatively slow step compared to CO dissociation. The combined transient kinetic data are used to fit an explicit microkinetic model for the methanation reaction. The mechanism involving direct CO dissociation represents the data better than a mechanism in which H-assisted CO dissociation is assumed. Microkinetics simulations based on the fitted parameters confirms that under methanation conditions the overall CO consumption rate is mainly controlled by C hydrogenation and to a smaller degree by O hydrogenation and CO dissociation. These simulations are also used to explore the influence of CO and H2 partial pressure on possible rate-controlling steps.

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

One of the well-known observations in the Fischer–Tropsch (FT) reaction is that the CH4 selectivity for cobalt catalysts is always higher than the value expected on the basis of the Anderson–Schulz–Flory (ASF) distribution. Depositing graphitic carbon on a cobalt catalyst strongly suppresses this non-ASF CH4, while the formation of higher hydrocarbons is much less affected. Carbon was laid down on the cobalt catalyst via the Boudouard reaction. We provide evidence that the amorphous carbon does not influence the FT reaction, as it can be easily hydrogenated under reaction conditions. Graphitic carbon is rapidly formed and cannot be removed. This unreactive form of carbon is located on terrace sites and mainly decreases the CO conversion by limiting CH4 formation. Despite nearly unchanged higher hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth probability and strongly suppresses olefin hydrogenation. We demonstrate that graphitic carbon will slowly deposit on the cobalt catalysts during CO hydrogenation, thereby influencing CO conversion and the FT product distribution in a way similar to that for predeposited graphitic carbon. We also demonstrate that the buildup of graphitic carbon by 13CO increases the rate of C–C coupling during the 12C3H6 hydrogenation reaction, whose products follow an ASF-type product distribution of the FT reaction. We explain these results by a two-site model on the basis of insights into structure sensitivity of the underlying reaction steps in the FT mechanism: carbon formed on step-edge sites is involved in chain growth or can migrate to terrace sites, where it is rapidly hydrogenated to CH4. The primary olefinic FT products are predominantly hydrogenated on terrace sites. Covering the terraces by graphitic carbon increases the residence time of CHx intermediates, in line with decreased CH4 selectivity and increased chain-growth rate.

On the Role of Acidity in Bulk and Nanosheet [T]MFI (T=Al3+, Ga3+, Fe3+, B3+) Zeolites in the Methanol-to-Hydrocarbons Reaction

The influence of framework substituents (Al3+, Ga3+, Fe3+ and B3+) and morphology (bulk vs. nanometer‐sized sheets) of MFI zeolites on the acidity and catalytic performance in the methanol‐to‐hydrocarbons (MTH) reaction was investigated. The Brønsted acid density and strength decreased in the order Al(OH)Si>Ga(OH)Si>Fe(OH)Si≫B(OH)Si. Pyridine 15N NMR spectra confirmed the differences in the Brønsted and Lewis acid strengths but also provided evidence for site heterogeneity in the Brønsted acid sites. Owing to the lower efficiency with which tervalent ions can be inserted into the zeolite framework, sheet‐like zeolites exhibited lower acidity than bulk zeolites. The sheet‐like Al‐containing MFI zeolite exhibited the greatest longevity as a MTH catalyst, outperforming its bulk [Al]MFI counterpart. Although the lower acidity of bulk [Ga]MFI led to a better catalytic performance than bulk [Al]MFI, the sheet‐like [Ga]MFI sample was found to be nearly inactive owing to lower and heterogeneous Brønsted acidity. All Fe‐ and B‐substituted zeolite samples displayed very low catalytic performance owing to their weak acidity. Based on the product distribution, the MTH reaction was found to be dominated by the olefins‐based catalytic cycle. The small contribution of the aromatics‐based catalytic cycle was larger for bulk zeolite than for sheet‐like zeolite, indicating that shorter residence time of aromatics can explain the lower tendency toward coking and enhanced catalyst longevity.

Direct synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure directing agent for methanol-to-hydrocarbons conversion

Hierarchical ZSM-5 zeolite can be obtained in a one-step synthesis approach using cetyltrimethyl–ammonium (CTA) as the sole organic template. The reduced crystal domain size and the presence of mesopores result in improved catalytic performance in methanol-to-hydrocarbon (MTH) reaction as compared to bulk ZSM-5. We investigated the role of the base (LiOH, NaOH, KOH, RbOH, CsOH), the H2O/CTA ratio, the Si/Al ratio and counter-ion of CTA (OH−vs. Br−). The crucial role of KOH and RbOH is evident as only these bases allow dissolution of the amorphous silica–alumina precursor to such extent that zeolite crystallization occurs. With other bases, silica dissolution is too limited to start zeolite crystallization, corroborated by the observation that seeding the synthesis gel rapidly led to mesoporous ZSM-5 zeolite for all bases. With KOH, mesoporous zeolite was obtained in the H2O/CTAOH 800–3200 range. The role of Al is also important as without it only ZSM-48 zeolite could be formed, while a too high Si/Al ratio of 20 did not lead to nucleation. A highly crystalline, hierarchical ZSM-5 free from extraframework Al synthesized from a gel with KOH as base, H2O/CTAOH and Si/Al ratios of 800 and 50, respectively displayed the highest catalytic performance in the MTH reaction, outperforming bulk ZSM-5.

Synthesis of stable and low-CO2 selective ε-iron carbide Fischer-Tropsch catalysts

Phase-pure ε-iron carbide catalysts amenable to scale-up exhibit low CO2 selectivity and high stability in the FT reaction. The Fe-catalyzed Fischer-Tropsch (FT) reaction constitutes the core of the coal-to-liquids (CTL) process, which converts coal into liquid fuels. Conventional Fe-based catalysts typically convert 30% of the CO feed to CO2 in the FT unit. Decreasing the CO2 release in the FT step will reduce costs and enhance productivity of the overall process. In this context, we synthesize phase-pure ε(′)-Fe2C catalysts exhibiting low CO2 selectivity by carefully controlling the pretreatment and carburization conditions. Kinetic data reveal that liquid fuels can be obtained free from primary CO2. These catalysts displayed stable FT performance at 23 bar and 235°C for at least 150 hours. Notably, in situ characterization emphasizes the high durability of pure ε(′)-Fe2C in an industrial pilot test. These findings contribute to the development of new Fe-based FT catalysts for next-generation CTL processes.