CO Dissociation Mechanism Research Articles

A Review of Theoretical Studies on Carbon Monoxide Hydrogenation via Fischer-Tropsch Synthesis over Transition Metals.

The increasing demand for clean fuels and sustainable products has attracted much interest in the development of active and selective catalysts for CO conversion to desirable products. This review maps the theoretical progress of the different facets of most commercial catalysts, including Co, Fe, Ni, Rh, and Ru. All relevant elementary steps involving CO dissociation and hydrogenation and their dependence on surface structure, surface coverage, temperature, and pressure are considered. The dominant Fischer-Tropsch synthesis mechanism is also explored, including the sensitivity to the structure of H-assisted CO dissociation and direct CO dissociation. Low-coordinated step sites are shown to enhance catalytic activity and suppress methane formation. The hydrogen adsorption and CO dissociation mechanisms are highly dependent on the surface coverage, in which hydrogen adsorption increases, and the CO insertion mechanism becomes more favorable at high coverages. It is revealed that the chain-growth probability and product selectivity are affected by the type of catalyst and its structure as well as the applied temperature and pressure.

Kinetics behavior of Co/Ni-ordered mesoporous alumina for the CO methanation

The severe requirement of a higher activity at lower temperatures and a longer stability at higher temperatures evokes a great challenge for the development of an industrially viable catalyst for the CO methanation reaction. In this work, the Co-Ni bimetallic catalysts were synthesized via post-impregnating the cobalt precursor within the mesoporous channel of Ni-embedded ordered mesoporous alumina (Ni-OMA). The low-temperature activity and high-temperature stability of Co/Ni-OMA for the CO methanation were significantly regulated by easily tuning the ratio of free Co and confined Ni species. The optimal catalyst of 8Co/15Ni-OMA showed a high activity with the CH4 formation rate of 126 mol kgcat−1 h−1 at a temperature of as low as 300 °C and a long-term durability for a time-on-stream of 200 h without an observable deactivation under the conditions of 600 °C and an extremely high GHSV of 180000 mL g−1 h−1. Kinetics results reveal that the apparent activation energy of the CO methanation over 8Co/15Ni-OMA (100.2 kJ mol−1) was clearly lower than that over 15Ni-OMA (124.0 kJ mol−1) or 15Co-OMA (131.8 kJ mol−1). In the absence of mass transport and heat transfer limitations, three microkinetics models were developed following the H-assisted CO dissociation and Langmuir-Hinshelwood mechanism, which the H-assisted CO dissociation, the hydrogenation of surface carbon species (C*) or surface CH3* species are proposed as the rate-determining step, respectively. The kinetics behaviors over 15Ni-OMA and 8Co/15Ni-OMA are matched well with all of the kinetics models, indicating the same elementary sequence and rate-determining step. In the case of 15Co/OMA, the CH4 formation rate was predicted very well by the kinetics models derived from the stepwise hydrogenation of surface carbon species as the rate-determining step, and the kinetics model based on the H-assisted CO dissociation as the rate-determining step could be ruled out, indicating that the rate for the H-assisted CO dissociation rate is faster than that of the following stepwise hydrogenation. Based on the discrimination of different kinetics models, Ni species confined within OMA matrix were proposed as the dominant active sites for catalyzing the CO methanation, while the post-impregnated Co was acted as a promoter for the H-assisted CO dissociation. As a result, an enhanced low-temperature activity was achieved over the optimal 8Co/15Ni-OMA catalyst.

Modeling Fischer–Tropsch kinetics and product distribution over a cobalt catalyst

AbstractA detailed kinetic model describing the consumption of key components and product distribution in the Fischer–Tropsch synthesis (FTS) over a 20%Co/0.5Re γ‐Al2O3 commercial catalyst is developed. The developed model incorporates the H2O‐assisted CO dissociation mechanism developed by Rytter and Holmen and a novel approach to product distribution modeling. The model parameters are optimized against an experimental dataset comprising a range of process conditions: total pressure 2.0–2.2 MPa, temperature 210–230°C, CO conversion range of 10%–75% and feed with and without added water. The quality of the model fit measured in terms of mean absolute relative residuals (MARR) value is 23.1%, which is comparable to literature reported values. The developed model can accurately describe both positive and negative effects of water on the rate kinetics, the positive effect of water on the growth factor, temperature and syngas composition on the kinetics and product distribution over a wide range of process conditions, which is critical for the design and optimization of the Fisher–Tropsch reactors.

Elucidating the Role of Bifunctional Cobalt‐Manganese Catalyst Interactions for Higher Alcohol Synthesis

Catalyst promoters are often used to tune or improve performance with little understanding as to how they work. Here we present a fundamental evaluation of cobalt‐manganese interactions in Fischer‐Tropsch (FT) catalysis and evaluate a new bifunctional catalyst for CO dissociation and CO insertion mechanisms. FT has gained considerable attention recently due to the potential to convert bio or municipal waste feeds into commercial fuels and chemicals. Products are commonly highly linear paraffins, but here we show how the role of manganese can tune selectivity for linear olefins and alcohols in a copper, iron and alkali metal free cobalt bifunctional catalyst. These products also have significant commercial value for lubricants, plasticizers, detergents and base chemicals. Advanced catalyst characterization is shown with in situ techniques (XRD, EXAFS, PDF & TEM), while the FT products are fully analysed by NMR, GC and GCxGC. The role of manganese during catalyst synthesis is reviewed while fundamental understanding of the formation of a mixed metal oxide spinel is presented. The Co–Mn interactions change during catalyst reduction, with EXAFS showing cobalt metal and MnO as the main species present.

CO Dissociation Mechanism on Mn-Doped Fe(100) Surface: A Computational Investigation

Periodic density function theory (DFT) and kinetic Monte Carlo (kMC) method are carried out to investigate CO dissociation process on the Mn-doped Fe(100) surface. The energetics information of relevant atomistic processes and adsorption features of relevant species are obtained from DFT calculations. Subsequently, kMC simulations are performed with DFT results employed as database. Simulations show that the energy barriers for CHO and COH formations are 0.09 eV and 0.35 eV larger than that for direct CO dissociation on Mn/Fe(100), respectively. An empty site is created with a CO hydrogenation (CO* + H* → COH* + *, CO* + H* → CHO* + *), while an active site is consumed with a CO direct dissociation (CO* + * → C* + O*). The number of unoccupied active sites can affect the way of CO dissociation. On surfaces with considerable unoccupied active sites, direct CO dissociation mechanism is the preferred route. Under conditions favoring a very low number of unoccupied active sites and a mass of adsorbed H on surfaces, H-assisted CO dissociation via COH will take place.

A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of ϴ-Fe3C

Abstract Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 1 1 ¯ 1 , and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the 0 1 ¯ 1 , (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the ( 0 1 ¯ 1 ) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.

Quantum-Chemical DFT Study of Direct and H- and C-Assisted CO Dissociation on the χ-Fe5C2 Hägg Carbide.

The first step in the Fischer–Tropsch reaction is the production of C1 monomers by the dissociation of the C–O bond. Although Fe is the active metal, it is well known that under typical reaction conditions, it changes into various carbide phases. The Hägg carbide (χ-Fe5C2) phase is usually considered as the catalytically active phase. We carried out a comprehensive DFT study of CO dissociation on various surface terminations of the Hägg carbide, selected on their specific site topology and the presence of stepped sites. Based on the reaction energetics, we identified several feasible CO dissociation pathways over the Hägg carbide. In this study, we have compared the direct CO dissociation with H- and C-assisted CO dissociation mechanisms. We demonstrated that the reaction rate for CO dissociation critically depends on the presence and topology of interstitial C atoms close to the active site. Typically, the CO dissociation proceeds via a direct C–O bond scission mechanism on the stepped sites on the Fe carbide surface. We have shown a preference for the direct CO dissociation on the surfaces with a stepped character. The H-assisted CO dissociation, via a CHO intermediate, was preferred when the surface did not have a clear stepped character. We have also shown that activation barriers for dissociation are highly dependent on the surface termination. With a consistent data set and including migration corrections, we then compared the CO dissociation rates based on a simplified kinetic model. With this model, we showed that besides the activation energy, the adsorption energy of the CO, the C and the O species are important for the reaction rate as well. We found that the most active surface termination is a (111̅) surface cut in such a way that the surface exposes B5 sites that are not occupied by the C atoms. On these B5 sites, the direct CO dissociation presents the highest rate.

The Effect of Cu Impurity on the CO-dissociation Mechanism on the Fe (100) Surface: A Full Potential DFT Study

In this study, the theoretical calculations of CO dissociation were carried out on Cu-Fe alloy surface by a full-potential method, which made more accurate results especially on the prediction of adsorption energies. This process may be governed by either a direct route or a H-assisted via HCO and COH intermediates pathways. In comparison to the pure surface Fe (100), the presence of Cu atom enhanced the activation energy of direct dissociation pathway and decreased that one for H-assisted mechanism through the HCO intermediate by 0.2eV and 0.24eV respectively. These effects were explained completely by the ligand effect and the ensemble effect corresponding to the electronic and geometrical activity of an adsorption-site on different states (initial, intermediate, and final).In this case, Cu impurity atom creates 3-fold-Fe-hollow site instead of 4-fold–Fe-hollow site on the surface as ensemble effect and Fe atoms near the Cu atom show more reactivity according to the ligand effect. The results showed that while both methods, CO-direct dissociation and H-assisted, are feasible on Fe (100) surface, only the later one happens on Fe-Cu surface. The results of this study suggest that Fe-Cu surface with moderate trend for adsorption has a higher life expectancy compared to Fe (100) due to the higher CO-direct dissociation-activation energy to build carbide. Also, by changing the main route from direct to H-assisted, the activity of Fischer- Tropsch reaction is enhanced with lower activation energy versus that one on Fe (100) surface.

CO adsorption, dissociation and coupling formation mechanisms on Fe2C(001) surface

By means of density functional theory calculations and atomic thermodynamics, we systematically investigated the CO adsorption on the Fe2C(001) surface at different coverage. It has been found that CO prefers to adsorb on the surface iron atom at low coverage (1–8 CO); CO prefers to adsorb at the bridge site of Fe and C atoms at high coverage (9–12 CO). Eight CO molecules binding on the Fe2C(001) surface is favorable thermodynamically as indicated by the stepwise adsorption energy. The phase diagram shows that addition of more CO molecules up to a number of 8 is thermodynamically favorable, and that the incremental energy gained by adding one more CO molecule is almost constant up to 4 CO molecules, decreases up to 8 CO molecules, after which it becomes thermodynamically unfavorable to add more CO molecules. Probability distribution of different singe-CO adsorbed states on the Fe2C(001) surface as function of temperature shows that CO dissociation and coupling are least preferred, indicating that carbide mechanism is not dominant in the iron-based Fischer-Tropsch synthesis reaction. The projected density of states (PDOS) was used to analyze the CO adsorption mechanism.

CO Dissociation Mechanism on Pd-Doped Fe(100): Comparison with Cu/Fe(100)

Spin-polarized density functional theory computations have been used to investigate the CO dissociation mechanisms and the different catalytic activities of the reaction on Fe(100) surfaces with different Pd coverages. CO can dissociate on Pd/Fe surfaces via three different mechanisms: direct and H-assisted mechanisms via HCO intermediate or COH intermediate. In our calculation, it was found that the activation barriers of direct CO and COH dissociation mechanisms on pure and Pd-doped Fe(100) surfaces were higher than that of the HCO dissociation mechanism. Besides, energy barriers for the identical reaction pathway on Fe-rich Fe(100) surfaces were lower than those on Pd-rich Fe(100) surfaces, namely, CO dissociation mainly occurs via the HCO intermediate pathway and the catalytic activity becomes lower with Pd coverage increasing toward CO dissociation in both direct CO and H-assisted CO dissociation mechanisms. As a result, CO dissociation mainly occurs on Fe-rich Pd/Fe surfaces, leading to the formatio...

A density functional theory study on the carbon chain growth of ethanol formation on Cu-Co (111) and (211) surfaces

Calculations based on the first-principle density functional theory were carried out to study the most controversial reactions in ethanol formation from syngas on Cu-Co surfaces: CO dissociation mechanism and the key reactions of carbon chain growth of ethanol formation (HCO insertion reactions) on four model surfaces (Cu-Co (111) and (211) with Cu-rich or Co-rich surfaces) to investigate the synergy of the Cu and Co components since the complete reaction network of ethanol formation from syngas is a huge computational burden to calculate on four Cu-Co surface models. We investigated adsorption of important species involved in these reactions, activation barrier and reaction energy of H-assisted dissociation mechanism, directly dissociation of CO, and HCO insertion reactions (CHx+HCO→CHxCHO (x=1–3)) on four Cu-Co surface models. It was found that reactions on Cu-rich (111) and (211) surfaces all have lower activation barrier in H-assisted dissociation and HCO insertion reactions, especially CH+HCO→CHCHO reaction. The PDOS of 4d orbitals of surface Cu and Co atoms of all surfaces were studied. Analysis of d-band center of Cu and Co atoms and the activation barrier data suggested the correlation between electronic property and catalytic performance. Cu-Co bimetallic with Cu-rich surface allows Co to have higher catalytic activity through the interaction of Cu and Co atom. Then it will improve the adsorption of CO and catalytic activity of Co. Thus it is more favorable to the carbon chain growth in ethanol formation. Our study revealed the factors influencing the carbon chain growth in ethanol production and explained the internal mechanism from electronic property aspect.

Mechanism of coverage dependent CO adsorption and dissociation on the Mo(100) surface.

The mechanism of coverage dependent CO adsorption and dissociation on the Mo(100) surface was investigated using periodic density functional theory. Structure optimization and frequency calculation were carried out using the GGA-PBE method and a p(3 × 3) supercell model. Energetic data have been obtained using the revised PBE method and the PBE optimized structures. CO adsorption prefers tilted adsorption configuration at the 4-fold hollow sites at low coverage and tilted and atop configurations at high coverage. The computed C-O stretching frequencies of the tilted and atop adsorbed CO molecules agree very well with the experimental results. Starting from the saturation coverage, five binding states have been found: two for molecular (α) CO adsorption and three for dissociative (β) CO adsorption, which are in agreement with the temperature-programmed desorption experiments. In addition, CO prefers dissociation with very low barriers in all coverages as long as free sites are available and is coverage independent; this nicely explains the observed CO dissociation at very low temperatures. All such agreements validate our computational methods and provide the basis of further studies.

Coverage-Dependent CO Adsorption and Dissociation Mechanisms on Iron Surfaces from DFT Computations

CO adsorption structures and energetics on the iron (100), (110), (111), (210), (211), and (310) surfaces from the lowest coverage up to saturation have been computed using spin-polarized density functional theory and ab initio thermodynamics. It is found that different adsorption configurations on each of these surfaces at high coverage can coexist. The stepwise adsorption energies and dissociation barriers at different coverage reveal equilibriums between desorption and dissociation of adsorbed CO molecules. Only molecular CO adsorption is possible at very high coverage and only dissociative CO adsorption at very low coverage, whereas mixed molecular and dissociative CO adsorption becomes possible at medium coverage. The computed stable adsorption configurations and the respective C–O and Fe–C stretching frequencies as well as desorption temperatures on the (100), (110), and (111) surfaces agree very well with the available experimental data. Such agreements between theory and experiment validate our co...

Preparation of Fe-Mn/K/Al2O3 Fischer-Tropsch Catalyst and Its Catalytic Kinetics for the Hydrogenation of Carbon Monoxide

A K promoted iron-manganese catalyst was prepared by sol-gel method, and subsequently was tested for hydrogenation of carbon monoxide to light olefins. The kinetic experiments on a well-characterized Fe-Mn/K/Al2O3 catalyst were performed in a fixed-bed micro-reactor in a temperature range of 280-380 °C, pressure range of 0.1-1.2 MPa, H2/CO feed molar ratio range of 1-2.1 and a space velocity range of 2000-7200 h−1. Considering the mechanism of the process and Langmuir-Hinshelwood-Hogan-Watson (LHHW) approach, unassisted CO dissociation and H-assisted CO dissociation mechanisms were defined. The best models were obtained using non-linear regression analysis and Levenberg-Marquardt algorithm. Consequently, 4 models were considered as the preferred models based on the carbide mechanism. Finally, a model was proposed as a best model that assumed the following kinetically relevant steps in the iron-Fischer-Tropsch (FT) synthesis: (1) CO dissociation occurred without hydrogen interaction and was not a rate-limiting step; (2) the first hydrogen addition to surface carbon was the rate-determining steps. The activation energy and adsorption enthalpy were calculated 40.0 and −30.2 kJ·mol−1, respectively.

A new insight into the initial step in the Fischer–Tropsch synthesis: CO dissociation on Ru surfaces

In the present work, we have investigated the CO dissociation on corrugated Ru(1121) and the stepped Ru(0001) surfaces by means of density functional theory with slab models. Our results show that, while the direct CO dissociation is preferred on the six-fold site of Ru(1121), the H-assisted CO dissociation is found to be favored on the B5 site of the stepped Ru(0001) surface. Furthermore, we have studied the effects of co-adsorbed spectator species on the CO dissociation mechanisms. Our results demonstrate that spectators can change the potential energy landscape dramatically, such that different reaction mechanisms can be favored in the presence of different spectators. Neither the H-assisted CO dissociation mechanism nor the CO direct dissociation mechanism should be overlooked at authentic ambient conditions. This work emphasizes a dynamic picture of the reaction mechanisms due to the inherent structural and compositional inhomogeneity on surfaces. Different mechanisms can work together as different active sites will co-exist on a real catalyst surface, and the reaction preferences on an active site can vary as the adsorbate compositions on surfaces are varying during the course of the reactions.

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

In the Fischer–Tropsch synthesis (FT) process the mechanism of CO dissociation is of fundamental importance. In this study we compare hydrogen-assisted CO dissociation mechanisms to direct CO dissociation on the B5 site as exposed on the fcc-Co(211) surface. Whereas direct CO dissociation is calculated to have an overall barrier of 142 kJ mol−1, the alternative mechanism involving a HCO intermediate proceeds with a lower overall barrier of 123 kJ mol−1. Using these calculated values we show that hydrogen-assisted CO dissociation will result in an overall rate corresponding to the same order as the considered FT experiments.

CO Dissociation Mechanism in Racemization of Alcohols by a Cyclopentadienyl Ruthenium Dicarbonyl Catalyst

(13)CO exchange studies of racemization catalyst (η(5)-Ph(5)C(5))Ru(CO)(2)Cl and (η(5)-Ph(5)C(5))Ru(CO)(2)(Ot-Bu) by (13)C NMR spectroscopy are reported. CO exchange for the active catalyst form, (η(5)-Ph(5)C(5))Ru(CO)(2)(Ot-Bu) is approximately 20 times faster than that for the precatalyst (η(5)-Ph(5)C(5))Ru(CO)(2)Cl. An inhibition on the rate of racemization of (S)-1-phenylethanol was observed on addition of CO. These results support the hypothesis that CO dissociation is a key step in the racemization of sec-alcohols by (η(5)-Ph(5)C(5))Ru(CO)(2)Cl, as also predicted by DFT calculations.

Supports for Mechanisms

Chemistry The mechanism of the low-temperature oxidation of CO by gold nanoclusters on oxide supports could depend on whether the oxide is a reducible metal, such as iron or titanium, or a nonreducible one, such as zinc. Carley et al. studied the oxidation of CO by gold nanoparticles supported on Fe2O3 and ZnO. The catalysts were exposed to a transient pulse of reactants (CO and O2) in an argon carrier at room temperature. When the catalysts were predosed with 18O-labeled water to form labeled surface hydroxyl groups, the initial CO2 product that formed on the zinc oxide support carried all three possible isotope distributions, but on iron oxide support, the initial CO2 product was fully 18O-labeled. These results are consistent with the formation of a nondissociative bicarbonate intermediate on the zinc oxide support, contrasting with a CO dissociation mechanism on iron oxide, where the adsorbed carbon reacts with surface hydroxyls. This dissociative mechanism is supported by x-ray photoelectron spectra, which showed the formation of adsorbed carbon after exposure to CO and O2 (but not CO alone). Density functional theory calculations show that oxidation of the gold atoms at the edge of the adsorbed clusters, along with the formation of surface bonds, can compensate for the high energy input needed to cleave CO and O2. Phys. Chem. Chem. Phys. 13 , 10.1039/c0cp01852j (2011).

Studies and Discriminations of the Kinetic Models for the Iron-Based Fischer−Tropsch Catalytic Reaction in a Recycle Slurry Reactor

The Fischer−Tropsch synthesis kinetic rates were studied by discussing the hydrogenation steps of the favored hydrogen-assisted CO dissociation mechanism and the surface site balance over an industrial iron-based catalyst. Their mathematical expressions are the two proposed models −rFT = (kFTPCOPH2α)/(kCOPCO + ∑i=1ixi) and −rFT = (kFTPCOPH2α)/((kCOPCO + ∑i=1ixi)2) in the literature. Unresolved relationships and differences between the two models are addressed here and analyzed specifically. When the significance of each of the terms in the two rival models is critically evaluated, both surface site balances of the two could be simplified to two involved terms: an adsorbed CO term and a vacant site term. Although the two models could hardly be distinguished from each other at the typical range of experimental conditions, further theoretical analyses and extrapolated experimental data favor the latter [its simplified form is −rFT = (kFTPCOPH20.5)/((1 + kCOPCO)2)]. These studies are helpful to understand the CO activation processes, and some contradictory conclusions and experimental phenomena in the literature can be explained consistently and reasonably.

Direct versus Hydrogen-Assisted CO Dissociation

The mechanism of CO dissociation is a fundamental issue in the technologically important Fischer-Tropsch (F-T) process that converts synthesis gas into liquid hydrocarbons. In the present study, we propose that on a corrugated Ru surface consisting of active sixfold (i.e., fourfold + twofold) sites, direct CO dissociation has a substantially lower barrier than the hydrogen-assisted paths (i.e., via HCO or COH intermediates). This proves that the F-T process on corrugated Ru surfaces and nanoparticles with active sixfold sites initiates through direct CO dissociation instead of hydrogenated intermediates.