First-principles based microkinetic modeling of transient kinetics of CO hydrogenation on cobalt catalysts

doi:10.1016/j.cattod.2019.03.002

Catalysis Today

Volume 342, 15 February 2020, Pages 131-141

https://doi.org/10.1016/j.cattod.2019.03.002 Get rights and content

Highlights

•
Density functional theory and microkinetics combined to model CO hydrogenation.
•
CO dissociation occurs in a direct fashion exclusively on step-edge sites.
•
CO hydrogenation rate control shared between CO bond scission and CH_x hydrogenation.
•
Experimental transient observations can be reproduced by microkinetic modeling.
•
Changing surface coverages strongly affect the reaction rate.

Abstract

Computational efforts towards a fundamental understanding of the underlying mechanistic pathways in synthesis gas conversion processes such as Fischer-Tropsch synthesis are exemplary for the developments in heterogeneous catalysis. Advances in transient kinetic analysis methods contribute to unraveling complex reaction pathways over nanoparticle surfaces. Tracing the activity and selectivity of Fischer-Tropsch catalysts to the individual events occurring at the active site remains difficult with experimental techniques. Here we provide simulations of transient kinetics at the scale of the active site by making use of the reaction energetics for CO hydrogenation to methane on stepped and terrace cobalt surfaces that are suitable models for cobalt FT nanoparticle catalysts. We investigate the hydrogen-deuterium kinetic isotope effect and simulate common steady-state and chemical isotopic transients. Comparison to experimental literature leads to important mechanistic insights. Direct CO dissociation is the main pathway for breaking the C single bond O bond and it occurs exclusively on step-edge sites. While the experimentally observed hydrogen-deuterium kinetic isotopic effect is often used as evidence for H-assisted CO dissociation, we show that hydrogenation of C and O as partly rate-controlling steps provides an alternative explanation. The simulations of the chemical transients provide significant insight into the importance of the changing surface coverages that strongly affect the reaction rate. The reversibility of CO dissociation on cobalt step-edges is evident from simulations of ¹²C¹⁶O/¹³C¹⁸O scrambling being in good agreement with experimental data.

Graphical abstract

Introduction

Gas-to-liquid processes like Fischer-Tropsch (FT) synthesis comprise alternative pathways from carbonaceous feedstock towards clean fuels and chemicals [1,2]. Considerable efforts have already been made to gain a deep understanding of the operation of relevant catalysts for the hydrogenation of carbon monoxide. While from the practical side catalysts are optimized with respect to mass and heat transport [3], catalyst attrition [4], and catalyst deactivation at high conversion [5], the focus at the fundamental level is on resolving the complex machinery of the elementary reactions that occur at the surface of supported metal nanoparticle catalysts.

The FT reaction is a chain reaction involving CH_x species as monomers. Unique initiation, propagation and termination rates lead to varying hydrocarbon product distributions on cobalt [6], ruthenium [7], and iron carbides [8]. For cobalt and ruthenium based catalysts, the initiation and propagation steps that give rise to the formation of long-chain hydrocarbons are usually assumed to proceed via the carbide mechanism [[9], [10], [11], [12]], i.e. the carbon-oxygen bond is broken prior to carbon-carbon coupling. However, the exact bond breaking mechanism remains heavily debated. The high activity of step-edge sites towards direct scission of π-bonds is widely accepted [[13], [14], [15], [16], [17], [18]], but the availability of such sites under CO hydrogenation conditions has been called into question due to carbon deposition [[19], [20], [21]] and lateral interactions [[22], [23], [24]]. Accordingly, hydrogen-assisted CO scission is often proposed as the dominant pathway in FT synthesis [17,[25], [26], [27]]. Chemical and isotopic transient kinetic experiments provide a powerful means to compare such mechanistic proposals [[28], [29], [30]]. Biloen and Sachtler [9] investigated surface intermediates with steady-state isotopic transient kinetic analysis (SSITKA) to develop their arguments in favor of the carbide mechanism. On the contrary, Kruse and coworkers proposed the importance of oxygenated intermediates to interpret chemical transient kinetic analysis (CTKA) measurements and emphasized CO insertion as the main growth mechanism [31,32]. The groups of Holmen and De Jong used these methods to determine CO coverages and found that CO residence times appeared independent of size for cobalt particles larger than 6 nm [[33], [34], [35], [36]]. Although these results indicate a reduced FT activity below 6 nm, the precise mechanism of chain growth on optimized larger particles remains a topic of controversy. Further insight can for instance be obtained through isotopic substitution of hydrogen with deuterium to determine kinetic isotope effects (KIE) [37]. A strong KIE in experiments using H₂ (r_H) and D₂ (r_D) can indicate a strong involvement of hydrogen in the controlling reaction steps of the mechanism. Inverse KIE ratios of r_H/r_D in the range of 0.7-0.8 have been associated with H-assisted CO dissociation routes [38,39]. However, such conclusions usually hinge on assumed rate-controlling steps, specifically CO dissociation. Hydrogen is also needed for termination of hydrocarbon species and removal of oxygen as water. Any rate control in these steps will therefore also influence the measured KIE ratios, regardless of the CO activation pathway. Unfortunately, experimental observations are usually a convolution of various pathways on different active sites. As such, direct deduction of the degree of rate control from experimental data is challenging [40,41]. Nevertheless, substantial efforts have been made to obtain this kind of information, predominantly by correlating predictions from microkinetic simulations to experimental data [[42], [43], [44], [45], [46], [47], [48], [49], [50]]. A corollary of these works is that catalytic activity is difficult to generalize using idealized models of isolated surface sites. Realistic descriptions of FT kinetics require a multi-site model that allows multiple reaction pathways. A specialized computer code developed for dealing with the complexity of SSITKA experiments has also been described in the literature as a realistic model leads to many options for isotopic compositions [51]. A salient result from this study is that the transient modeling results suggest a similar reaction mechanism over fcc and hcp cobalt with mainly differences in the number of active sites.

Here we report on the development of a microkinetic model to complement experimental transient studies of CO hydrogenation. The microkinetic simulations utilize a continuous stirred-tank reactor (CSTR) model to allow a changing gas-phase composition over time and all intermediates are explicitly labeled to follow isotopic changes. In this study all relevant elementary reaction steps are included from synthesis gas as a feedstock to methane, water, and carbon dioxide products. The reaction constants are computed from differences in energies and configurations of the reaction intermediates and transition states determined by density functional theory (DFT). As entropy is especially important for predicting accurate adsorption and desorption rates we incorporated gas-phase entropies from thermodynamic tables. With these contributions the desorption rates of CO and H₂ match experimental results well. The complex surface of cobalt nanoparticles is approximated by a mean field of terrace and step-edge sites in a 10:1 ratio. This site ratio is a conservative estimate of the abundance of step-edge sites on fcc cobalt nanoparticles of 6–8 nm. [52] As actual site compositions might differ we included a sensitivity analysis of this parameter in the supplementary information. The mechanistic trends in this study were found to be insensitive to both the site ratio and to the reaction barrier for migration between the different sites. Migration reactions between these sites are included to explain the results of site blocking experiments [53,54]. Simulated steady-state methanation with hydrogen and deuterium shows KIE ratios close to the experimental values and reveal step-edges as the active site for CO dissociation. Simulated isotopic and chemical transients then show the importance of coverage effects, further unraveling the initiation pathway in FT synthesis over cobalt nanoparticles.

Section snippets

Methods

All quantum-chemical calculations were performed using a plane-wave density functional theory approach with the projector-augmented wave (PAW) method [55], as implemented in the Vienna ab initio simulation package (VASP) [[56], [57], [58], [59]]. For exchange-correlation, the Perdew-Burke-Ernzerhof (PBE) functional was used [60]. The plane-wave basis set was limited to a 400 eV kinetic energy cutoff base. A gamma-centered Monkhorst-Pack mesh [61] k-points of (21 × 21 × 21) was used for bulk hcp

Results and discussion

We initially investigated the reaction energetics of the methanation reaction on the two cobalt surfaces by DFT calculations. The two surfaces and several transition states are depicted in Fig. 1. The surface of the step-edge Co $(11 \bar{2} 1)$ model can be described as a two-dimensional array of B₅-sites. The forward and backward activation barriers, corrected for the zero-point energy, are listed in Table 1. For CO dissociation, we considered both direct and H-assisted C single bond O bond scission. For the

Conclusion

Microkinetic simulations of transient kinetics for a cobalt nanoparticle model surface display strong evidence for CH_x formation through a direct CO scission pathway. Computed reaction energetics show that this pathway needs step-edge sites and does not take place on terraces. Thermodynamically consistent ab initio reaction energetics allows simulation of TPD profiles for CO and H₂ and can also reproduce the effect of lateral interactions. In SSITKA and CTKA simulations, we neglected

Declarations of interest

None.

Acknowledgements

The authors gratefully acknowledge financial support from Shell Global Solutions International B.V. The Netherlands Organization for Scientific Research is acknowledged for providing access to computational resources.

References (91)

M.E. Dry
The fischer-tropsch process - commercial aspects
Catal. Today
(1990)
M.E. Dry
The Fischer–Tropsch process: 1950–2000
Catal. Today
(2002)
B.C. Enger et al.
Modified alumina as catalyst support for cobalt in the Fischer–Tropsch synthesis
J. Catal.
(2011)
E. Iglesia
Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts
Appl. Catal. A Gen.
(1997)
D.L. King
A Fischer-Tropsch study of supported ruthenium catalysts
J. Catal.
(1978)
B.H. Davis
Fischer–tropsch Synthesis: reaction mechanisms for iron catalysts
Catal. Today
(2009)
P. Biloen et al.
On the activity of Fischer-Tropsch and methanation catalysts: a study utilizing isotopic transients
J. Catal.
(1983)
P. Biloen et al.
Incorporation of surface carbon into hydrocarbons during Fischer-Tropsch synthesis: mechanistic implications
J. Catal.
(1979)
R. Van Hardeveld et al.
The statistics of surface atoms and surface sites on metal crystals
Surf. Sci.
(1969)
R. van Hardeveld et al.
The influence of crystallite size on the adsorption of molecular nitrogen on nickel, palladium and platinum: an infrared and electron-microscopic study
Surf. Sci.
(1966)

D.J. Moodley et al.

Carbon deposition as a deactivation mechanism of cobalt-based Fischer–Tropsch synthesis catalysts under realistic conditions

Appl. Catal. A Gen.

(2009)

K. Fei Tan et al.

Carbon deposition on Co catalysts during Fischer–tropsch synthesis: a computational and experimental study

J. Catal.

(2010)

M. Zhuo et al.

Effect of the CO coverage on the Fischer–Tropsch synthesis mechanism on cobalt catalysts

J. Catal.

(2013)

D. Hibbitts et al.

Preferential activation of CO near hydrocarbon chains during Fischer–Tropsch synthesis on Ru

J. Catal.

(2016)

M. Ojeda et al.

CO activation pathways and the mechanism of Fischer–Tropsch synthesis

J. Catal.

(2010)

M.P. Andersson et al.

Structure sensitivity of the methanation reaction: H2-induced CO dissociation on nickel surfaces

J. Catal.

(2008)

H.A.J. van Dijk et al.

Steady-state isotopic transient kinetic analysis of the Fischer–Tropsch synthesis reaction over cobalt-based catalysts

Chem. Eng. Sci.

(2001)

J. Yang et al.

Particle size effect for cobalt Fischer–Tropsch catalysts based on in situ CO chemisorption

Surf. Sci.

(2016)

T.L. Wind et al.

Comparison of mechanistic understanding and experiments for CO methanation over nickel

J. Catal.

(2016)

E. Rebmann et al.

Kinetic modeling of transient Fischer–Tropsch experiments over Co/Al2O3 catalysts with different microstructures

Catal. Today

(2016)

P. van Helden et al.

The size-dependent site composition of FCC cobalt nanocrystals

Catal. Today

(2016)

W. Chen et al.

Silver addition to a cobalt Fischer–Tropsch catalyst

J. Catal.

(2018)

J.C. Kelzenberg et al.

Fischer-Tropsch synthesis over Ru-group Ib/SiO2 catalysts

J. Catal.

(1990)

G. Kresse et al.

Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

Comput. Mater. Sci.

(1996)

J. Lahtinen et al.

Adsorption and structure dependent desorption of CO on Co(0001)

Surf. Sci.

(1998)

C.J. Weststrate et al.

Spectroscopic insights into cobalt-catalyzed Fischer-Tropsch synthesis: a review of the carbon monoxide interaction with single crystalline surfaces of cobalt

J. Catal.

(2016)

G. Mills et al.

Reversible work transition state theory: application to dissociative adsorption of hydrogen

Surf. Sci.

(1995)

F. Abild-Pedersen et al.

CO adsorption energies on metals with correction for high coordination adsorption sites – a density functional study

Surf. Sci.

(2007)

G.T.K.K. Gunasooriya et al.

CO adsorption on cobalt: prediction of stable surface phases

Surf. Sci.

(2015)

C.S. Kellner et al.

Evidence for H2D2 isotope effects on fischer-tropsch synthesis over supported ruthenium catalysts

J. Catal.

(1981)

M.A. Logan et al.

Deuterium isotope effects on hydrogenation of carbon monoxide over rhodium

J. Catal.

(1985)

H. Matsumoto et al.

The transient method applied to the methanation and Fischer-Tropsch reactions over a fused iron catalyst

J. Catal.

(1978)

M. Lualdi et al.

Evidence for diffusion-controlled hydrocarbon selectivities in the Fischer–Tropsch synthesis over cobalt supported on ordered mesoporous silica

Top. Catal.

(2011)

M. Wolf et al.

Size dependent stability of cobalt nanoparticles on silica under high conversion Fischer–Tropsch environment

Faraday Discuss.

(2017)

R.A. van Santen et al.

Mechanism and microkinetics of the Fischer–Tropsch reaction

J. Chem. Soc. Faraday Trans.

(2013)

I.A.W. Filot et al.

Quantum chemistry of the Fischer–Tropsch reaction catalysed by a stepped ruthenium surface

Catal. Sci. Technol.

(2014)

J.X. Liu et al.

Crystallographic dependence of CO activation on cobalt catalysts: HCP versus FCC

J. Am. Chem. Soc.

(2013)

I.A.W. Filot et al.

Size and topological effects of rhodium surfaces, clusters and nanoparticles on the dissociation of CO

J. Phys. Chem. C

(2011)

S. Shetty et al.

Direct versus hydrogen-assisted CO dissociation

J. Am. Chem. Soc.

(2009)

X.Y. Quek et al.

Correlating Fischer-Tropsch activity to Ru nanoparticle surface structure as probed by high-energy X-ray diffraction

Chem. Commun. (Camb.)

(2014)

S.B. Vendelbo et al.

Self blocking of CO dissociation on a stepped ruthenium surface

Top. Catal.

(2010)

B.T. Loveless et al.

CO chemisorption and dissociation at high coverages during CO hydrogenation on Ru catalysts

J. Am. Chem. Soc.

(2013)

P. van Helden et al.

Hydrogen-assisted CO dissociation on the Co(211) stepped surface

Catal. Sci. Technol.

(2012)

C. Ledesma et al.

Recent approaches in mechanistic and kinetic studies of catalytic reactions using SSITKA technique

ACS Catal.

(2014)

S.L. Shannon et al.

Characterization of catalytic surfaces by isotopic-transient kinetics during steady-state reaction

Chem. Rev.

(1995)

Cited by (33)

Theoretical assessments of CO<inf>2</inf> activation and hydrogenation pathways on transition-metal surfaces
2023, Applied Surface Science
Carbon dioxide (CO₂) hydrogenation on transition-metal active sites offers a promising carbon utilization route toward mitigating greenhouse gas emissions. C₁ products are often formed in parallel during CO₂ hydrogenation, prompting investigations into the intrinsic properties of transition metals that drive activity and product selectivity. In this work, close-packed surfaces of a selection of transition-metal catalysts (Ni, Co, Rh, Ru, Pd, and Pt) were studied with density functional theory (DFT) calculations to understand their fundamental reactivities for CO₂ transformation reactions. Results indicate that CO₂ conversion proceeds through CO* formation and hydrogenation to form C₁ products (* denotes an adsorbed species). Ni, Co, Rh, and Ru favor CO/CH₄ formation, while Pd and Pt favor CO/CH₃OH formation. The ability of a metal to dissociate C-O bonds drives selectivity between CH₄ and CH₃OH, while competition between CO* desorption and surface hydrogenation describes CO selectivities. The C-O bond dissociation steps often impose the highest barrier along CH₄ formation reaction profiles, suggesting their kinetic relevance for CH₄ formation rates. The provided DFT-derived data sets detail a comprehensive reaction network of elementary steps relevant to C₁ chemistries, ultimately offering a benchmark for insights into design strategies for materials that exploit transition-metal active sites in carbon capture or utilization processes.
Iterative multiscale and multi-physics computations for operando catalyst nanostructure elucidation and kinetic modeling
2023, iScience
Modern heterogeneous catalysis has benefitted immensely from computational predictions of catalyst structure and its evolution under reaction conditions, first-principles mechanistic investigations, and detailed kinetic modeling, which are rungs on a multiscale workflow. Establishing connections across these rungs and integration with experiments have been challenging. Here, operando catalyst structure prediction techniques using density functional theory simulations and ab initio thermodynamics calculations, molecular dynamics, and machine learning techniques are presented. Surface structure characterization by computational spectroscopic and machine learning techniques is then discussed. Hierarchical approaches in kinetic parameter estimation involving semi-empirical, data-driven, and first-principles calculations and detailed kinetic modeling via mean-field microkinetic modeling and kinetic Monte Carlo simulations are discussed along with methods and the need for uncertainty quantification. With these as the background, this article proposes a bottom-up hierarchical and closed loop modeling framework incorporating consistency checks and iterative refinements at each level and across levels.
Recent advances in the use of Steady-State Isotopic Transient Kinetic Analysis data in (micro)kinetic modeling for catalyst and process design
2023, Catalysis Communications
Developing new catalysts with improved performance is crucial for establishing a sustainable chemical industry and requires atomic-scale knowledge of the relevant catalyst-species interactions. A powerful experimental technique to gather such detailed knowledge is Steady-State Isotopic Transient Kinetic Analysis (SSITKA). By combining SSITKA with (micro)kinetic modeling, the observed catalyst performance can be rationalized to advance the development of structure-activity relationships, and to obtain fundamental insights in complex reaction pathways. In turn, tailored experimental and computational methodologies provide the tools to access this detailed information. This mini review summarizes the developments that have been made in these fields during the last decade.
Equilibrium and kinetic isotope effects in heterogeneous catalysis: A density functional theory perspective
2023, Catalysis Communications
Isotopes are indispensable tools for elucidating catalytic reaction mechanisms. Yet, computational isotopic studies in heterogeneous catalysis remain scarce, especially when compared with other subfields of chemistry. In this review, we discuss isotope effects from a computational viewpoint, focusing specifically on heterogeneous catalysts. First, we review methods for predicting kinetic and equilibrium isotope effects of individual elementary steps, as well as for simulating the apparent kinetic isotope effects of multistep reactions. Second, we discuss the influence of the often-neglected catalytic environment, such as the presence of different binding sites and non-zero adsorbate coverages. Third, we present case studies illustrating possible pitfalls when performing isotope studies and how computations can help avoid them. Finally, we share our outlook on possible future applications of isotope effects. Throughout the review, we highlight the promising synergies that can be gained by combining experiments and computations.
The new role of pivotal intermediate CH<inf>x</inf> in CO activation and conversion to hydrocarbons on the transition metal catalysts
2023, Fuel
Generally, in the Fischer Tropsch synthesis experiments, the pivotal intermediate CH_x(x = 1–3) are only regarded as the reactive species to participate into the reaction, and the other functions of CH_x(x = 1–3) species are ignored. The experiment hardly identifies the microscopic function mechanism of CH_x(x = 1–3) formed over the Co-based catalysts due to the dynamic change of CH_x(x = 1–3) intermediate. Through DFT calculations and microkinetic modeling, this work illustrates the new mechanism of CH_x(x = 1–3) adsorbed over Co catalyst (CH_x(x = 1–3)/Co) to catalyze syngas-to-hydrocarbon for the first time. The produced CH_x not only participates into the reactions as the reactive intermediate, but also performs the role of surface promoter to enhance the activity and selectivity toward CH_x(x = 1–3) and C₂ hydrocarbons. CH as the main monomer has the best promotion effect. Interestingly, the same things also occur in CO methanation over CH_x(x = 1–3)/Ni catalysts, which is highly active and selective toward methane. Thus, the new mechanism of CH_x(x = 1–3) intermediate as the catalytic assist located over the transition metal may be popularized in the other reaction systems.
Dynamic operation of Fischer-Tropsch reactors for power-to-liquid concepts: A review
2022, Renewable and Sustainable Energy Reviews
The Fischer-Tropsch synthesis (FTS) is considered as a power-to-X (PtX) storage concept for converting temporally available excess energy to fuels or chemical compounds without the need of fossil resources. Fluctuating energy supplies demand a load-flexible energy system and a dynamically operating FTS reactor might be beneficial compared to traditional steady-state operations which rely on expensive upstream buffer capacities.
This review provides an overview of recent experimental and simulation studies dealing with dynamic FTS operation and summarizes the main findings. The results are presented the two categories, process intensification and PtX application. The review further discusses the experimentally difficult task of wide-ranging product characterization with a high temporal resolution.
While dynamic reactor operation is often related to a complicated process control, which challenges a save and efficient reactor performance, the literature findings indicate that for dynamic FTS operation such concerns might not be as critical as assumed, at least within well-known boundaries.
Researchers further agree that dynamic operation might be a tool for process intensification. Especially hydrogen pulsing seems to be a potentially beneficial operating technique to remove accumulated liquid products, restore initial catalyst activity and increase diesel-range productivity. The main challenge in this context is the prevention of high methane selectivity.
A lucid future engineering goal seems to be the combination of the two applications: a robust and reliable FTS reactor in a PtX scenario that not only handles a fluctuating feed but uses such variations for process enhancement.

View all citing articles on Scopus

View full text