Understanding the effect of cobalt particle size on Fischer-Tropsch synthesis: surface species and mechanistic studies by SSITKA and kinetic isotope effect

Structure Sensitivity of CO2 Hydrogenation on Ni Revisited

Despite the large number of studies on the catalytic hydrogenation of CO2 to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO2 hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO2 to CO are different from those for the subsequent hydrogenation of CO to CH4. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO2 hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO2 hydrogenation and demonstrated that direct dissociation of CO2 to CO is followed by the conversion of strongly bonded carbonyls to CH4. These findings provide essential insights into the much debated structure sensitivity of CO2 hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.

Size-Dependent Reactivity of Con- (n = 5-25) Cluster Anions toward Carbon Dioxide

A fundamental understanding of the reactivity evolution of nanosized clusters at an atomically precise level is pivotal to assemble desired materials with promising candidates. Benefiting from the tandem mass spectrometer coupled with a high-temperature ion-trap reactor, the reactions of mass-selected Con- (n = 5-25) clusters with CO2 were investigated and the increased reactivity of Co20-25- was newly discovered herein. This finding marks an important step to understand property evolution of subnanometer metal clusters (Co25-, ∼0.8 nm) atom-by-atom. The reasons behind the increased reactivity of Co20-25- were proposed by analyzing the reactions of smaller Co6-8- clusters that exhibit significantly different reactivity toward CO2, in which a lower electron affinity of Con contributes to the capture of CO2 while the flexibility of Con- could play vital roles to stabilize reaction intermediates and suppress the barriers of O-CO rupture and CO desorption.

Mechanistic Connections between CO2 and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Catalytic routes for upgrading CO₂ to CO and hydrocarbons have been studied for decades, and yet the mechanistic details and structure-function relationships that control catalytic performance have remained unresolved. This study elucidates the elementary steps that mediate these reactions and examines them within the context of the established mechanism for CO hydrogenation to resolve the persistent discrepancies and to demonstrate inextricable links between CO₂ and CO hydrogenation on dispersed Ru nanoparticles (6-12 nm mean diameter, 573 K). The formation of CH₄ from both CO₂-H₂ and CO-H₂ reactants requires the cleavage of strong C≡O bonds in chemisorbed CO, formed as an intermediate in both reactions, via hydrogen-assisted activation pathways. The C═O bonds in CO₂ are cleaved via direct interactions with exposed Ru atoms in elementary steps that are shown to be facile by fast isotopic scrambling of C¹⁶O₂-C¹⁸O₂-H₂ mixtures. Such CO₂ activation steps form bound CO molecules and O atoms; the latter are removed via H-addition steps to form H₂O. The kinetic hurdles in forming CH₄ from CO₂ do not reflect the inertness of C═O bonds in CO₂ but instead reflect the intermediate formation of CO molecules, which contain stronger C≡O bonds than CO₂ and are present at near-saturation coverages during CO₂ and CO hydrogenation catalysis. The conclusions presented herein are informed by a combination of spectroscopic, isotopic, and kinetic measurements coupled with the use of analysis methods that account for strong rate inhibition by chemisorbed CO. Such methods enable the assessment of intrinsic reaction rates and are essential to accurately determine the effects of nanoparticle structure and composition on reactivity and selectivity for CO₂-H₂ reactions.

Promotional effect of in situ generated hydroxyl on olefin selectivity of Co-catalyzed Fischer-Tropsch synthesis

The understanding of the water effect on olefin selectivity in Fischer-Tropsch synthesis (FTS) is limited by the complexity of the reaction network. Herein, we employ propene hydrogenation as a model reaction to isolate the water effect on olefin adsorption and hydrogenation from the complex reaction of FTS. It is clearly observed that the added water inhibits the activity of propene hydrogenation on two cobalt catalysts supported on high-surface-area alumina (HAS Al₂O₃) and low-surface-area alumina (LSA Al₂O₃), respectively. The inhibiting effect is much stronger for Co/HSA Al₂O₃. DFT investigation demonstrates that the in situ generated OH, rather than H₂O and O, impedes the adsorption of propene and thus decreases the activity of propene hydrogenation. The suppressive effect of OH on propene adsorption is attributed to the downshift of the d-band center and the Bader charge of the catalyst surface. The DFT-based kinetic analysis finds that the higher site coverage of OH results in the more pronounced negative effect on propene hydrogenation. Furthermore, the theory of OH-induced weak olefin adsorption and low olefin hydrogenation activity could rationalize the enhancement effect of water on the olefin selectivity and the particle size dependence of the water effect in FTS. The insights obtained here may inspire researchers to optimize olefin selectivity by manipulating the electronic properties of catalysts with hydroxyl species.

Investigation of C1 + C1 Coupling Reactions in Cobalt-Catalyzed Fischer-Tropsch Synthesis by a Combined DFT and Kinetic Isotope Study

Understanding the chain growth mechanism is of vital importance for the development of catalysts with enhanced selectivity towards long-chain products in cobalt-catalyzed Fischer-Tropsch synthesis. Herein, we discriminate various C1 + C1 coupling reactions by theoretical calculations and kinetic isotope experiments. CHx(x=0−3), CO, HCO, COH, and HCOH are considered as the chain growth monomer respectively, and 24 possible coupling reactions are first investigated by theoretical calculations. Eight possible C1 + C1 coupling reactions are suggested to be energetically favorable because of the relative low reaction barriers. Moreover, five pathways are excluded where the C1 monomers show low thermodynamic stability. Effective chain propagation rates are calculated by deconvoluting from reaction rates of products, and an inverse kinetic isotope effect of the C1 + C1 coupling reaction is observed. The theoretical kinetic isotope effect of CO + CH2 is inverse, which is consistent with the experimental observation. Thus, the CO + CH2 pathway, owing to the relatively lower barrier, the high thermodynamic stability, and the inverse kinetic isotope effect, is suggested to be a favorable pathway.

Preparation of Cobalt Nanocrystals Supported on Metal Oxides To Study Particle Growth in Fischer-Tropsch Catalysts

Colloidal synthesis of nanocrystals (NC) followed by their attachment to a support and activation is a promising route to prepare model catalysts for research on structure-performance relationships. Here, we investigated the suitability of this method to prepare well-defined Co/TiO₂ and Co/SiO₂ catalysts for the Fischer–Tropsch (FT) synthesis with high control over the cobalt particle size. To this end, Co-NC of 3, 6, 9, and 12 nm with narrow size distributions were synthesized and attached uniformly on either TiO₂ or SiO₂ supports with comparable morphology and Co loadings of 2–10 wt %. After activation in H₂, the FT activity of the TiO₂-supported 6 and 12 nm Co-NC was similar to that of a Co/TiO₂ catalyst prepared by impregnation, showing that full activation was achieved and relevant catalysts had been obtained; however, 3 nm Co-NC on TiO₂ were less active than anticipated. Analysis after FT revealed that all Co-NC on TiO₂ as well as 3 nm Co-NC on SiO₂ had grown to ∼13 nm, while the sizes of the 6 and 9 nm Co-NC on SiO₂ had remained stable. It was found that the 3 nm Co-NC on TiO₂ already grew to 10 nm during activation in H₂. Furthermore, substantial amounts of Co (up to 60%) migrated from the Co-NC to the support during activation on TiO₂ against only 15% on SiO₂. We showed that the stronger interaction between cobalt and TiO₂ leads to enhanced catalyst restructuring as compared to SiO₂. These findings demonstrate the potential of the NC-based method to produce relevant model catalysts to investigate phenomena that could not be studied using conventionally synthesized catalysts.

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 CH_x 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 CH_x hydrogenation rather than CO dissociation. Backward CO/H₂ → H₂ CTKA emphasizes the importance of H coverage on the slow CH_x 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.

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 CH_x 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.

Evidence of Structure Sensitivity in the Fischer-Tropsch Reaction on Model Cobalt Nanoparticles by Time-Resolved Chemical Transient Kinetics

The Fischer-Tropsch process, or the catalytic hydrogenation of carbon monoxide (CO), produces long chain hydrocarbons and offers an alternative to the use of crude oil for chemical feedstocks. The observed size dependence of cobalt (Co) catalysts for the Fischer-Tropsch reaction was studied with colloidally prepared Co nanoparticles and a chemical transient kinetics reactor capable of measurements under non-steady-state conditions. Co nanoparticles of 4.3 nm and 9.5 nm diameters were synthesized and tested under atmospheric pressure conditions and H₂ /CO=2. Large differences in carbon coverage (Θ_C ) were observed for the two catalysts: the 4.3 nm Co catalyst has a Θ_C less than one while the 9.5 nm Co catalyst supports a Θ_C greater than two. The monomer units present on the surface during reaction are identified as single carbon species for both sizes of Co nanoparticles, and the major CO dissociation site is identified as the B₅ -B geometry. The difference in activity of Co nanoparticles was found to be a result of the structure sensitivity caused by the loss of these specific types of sites at smaller nanoparticle sizes.

Understanding FTS selectivity: the crucial role of surface hydrogen

Monomeric forms of carbon play a central role in the synthesis of long chain hydrocarbons via the Fischer-Tropsch synthesis (FTS). We explored the chemistry of C₁H_xad species on the close-packed surface of cobalt. Our findings on this simple model catalyst highlight the important role of surface hydrogen and vacant sites for product selectivity. We furthermore find that CO_ad affects hydrogen in multiple ways. It limits the adsorption capacity for H_ad, lowers its adsorption energy and inhibits dissociative H₂ adsorption. We discuss how these findings, extrapolated to pressures and temperatures used in applied FTS, can provide insights into the correlation between partial pressure of reactants and product selectivity. By combining the C₁H_x stability differences found in the present work with literature reports of the reactivity of C₁H_x species measured by steady state isotope transient kinetic analysis, we aim to shed light on the nature of the atomic carbon reservoir found in these studies.

Revealing the Size-Dependent d-d Excitations of Cobalt Nanoparticles Using Soft X-ray Spectroscopy

Cobalt-based catalysts are widely used to produce liquid fuels through the Fischer-Tropsch (FT) reaction. However, the cobalt nanocatalysts can exhibit intriguing size-dependent activity whose origin remains heavily debated. To shed light on this issue, the electronic structures of cobalt nanoparticles with size ranging from 4 to 10 nm are studied using soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopies. The RIXS measurements reveal the significant size-dependent d-d excitations, from which we determine that the crystal-field splitting energy 10Dq changes from 0.6 to 0.9 eV when the particle size is reduced from 10 to 4 nm. The finding that larger Co nanoparticles have smaller 10Dq value is further confirmed by the Co L-edge RIXS simulations with atomic multiplet code. Our RIXS results demonstrate a stronger Co-O bond in smaller Co nanoparticles, which brings in further insight into their size-dependent catalytic performance.

Highly selective production of heavy hydrocarbons over cobalt–graphene–silica nanocomposite catalysts

Cobalt–graphene–silica nanocomposites catalysts were applied in FTS and showed highly selective production of heavy hydrocarbons.

Metallic nanocatalysis: an accelerating seamless integration with nanotechnology

Rapidly growing research interests surround heterogeneous nanocatalysis, in which metal nanoparticles (NPs) play a pivotal role as structure-sensitive active centers. With advances in nanotechnology, the morphology of metal NPs can be precisely controlled, which can provide well-defined models of nanocatalysts for understanding and optimizing the structure-reactivity correlations and the catalytic mechanisms. Benefiting from this, further credible evidence can be acquired on well-defined nanocatalysts rather than common multiphase systems, which is of great significance for the design and practical application of active metal nanocatalysts. Numerous studies demonstrate that enhanced structure-sensitive catalytic activity and selectivity are dependent not only on an increased surface-to-volume ratio and special surface atom arrangements, but also on tailored metal-metal and metal-organic-ligand interfaces, which is ascribed to the size, shape, composition, and ligand effects. Size-reactivity relationships and underlying size-dependent metal-oxide interactions are observed in many reactions. For bimetallic nanocatalysts, the composition and nanostructure play critical roles in regulating reactivities. Crystal facets favor individual catalytic selectivity and rates via distinct reaction pathways occurring on diverse atomic arrangements, both to low-index and high-index facets. High-index facets exhibit superior reactivities owing to their high-energy active sites, which facilitate rapid bond-breaking and new bond generation. Additionally, organic ligands may enhance the catalytic activity and selectivity of metal nanocatalysts via changing the adsorption energies of reactants and/or reaction energy barriers. Furthermore, atomically dispersed metals, especially single-atom metallic catalysts, have emerged recently, which can achieve better specific catalytic activity compared to conventional nanostructured metallic catalysts due to the low-coordination environment, stronger interaction with supports, and maximum service efficiency. Here, recent progress in shaped metallic nanocatalysts is examined and several parameters are discussed, as well as finally highlighting single-atom metallic catalysts and some perspectives on nanocatalysis. The integration of nanotechnology and nanocatalysis has been shaping up and, no doubt, the combination of sensitive characterization techniques and quantum calculations will play more important roles in such processes.

Mechanism and microkinetics of the Fischer-Tropsch reaction

The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer-Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer-Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer-Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C-O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer-Tropsch catalysis.

Discrimination of the mechanism of CH4 formation in Fischer–Tropsch synthesis on Co catalysts: a combined approach of DFT, kinetic isotope effects and kinetic analysis

The mechanism of CH₄ formation during Fischer–Tropsch synthesis on cobalt has been studied.