Mechanism Of CO Hydrogenation Research Articles

In Situ Active Site for CO Activation in Fe-Catalyzed Fischer-Tropsch Synthesis from Machine Learning.

In situ-formed iron carbides (FeCx) are the key components responsible for Fischer-Tropsch synthesis (FTS, CO + H2 → long-chain hydrocarbons) on Fe-based catalysts in industry. The true active site is, however, highly controversial despite more than a century of study, which is largely due to the combined complexity in both FeCx structures and mechanism of CO hydrogenation. Herein powered by machine learning simulation, millions of structure candidates for FeCx bulk and surfaces are explored under FTS conditions, which leads to resolving the active site for CO activation. This is achieved without a priori input from experiment by first constructing the thermodynamics convex hull of bulk phases, followed by identifying the low surface energy surfaces and evaluating the adsorption ability of CO and H, and finally determining the lowest energy reaction pathway of CO activation. Rich information on FeCx structures and CO hydrogenation pathways is gleaned: (i) Fe5C2, Fe7C3, and Fe2C are the three stable bulk phases under FTS in producing olefins, where Fe7C3 and Fe2C have multiple energetically nearly degenerate bulk crystal phases; (ii) only three low surface energy surfaces of these bulk phases, namely, χ-Fe5C2(510), χ-Fe5C2(111), and η-Fe2C(111), expose the Fe sites that can adsorb H atoms exothermically, where the surface Fe:C ratio is 2, 1.75, and 2, respectively; (iii) CO activation via direct dissociation can occur at the surface C vacancies (e.g., with a barrier of 1.1 eV) that are created dynamically via hydrogenation. These atomic-level understandings facilitate the building of the structure-activity correlation and designing better FT 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 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.

Effect of Co crystallinity on Co/CNT catalytic activity in CO/CO2 hydrogenation and CO disproportionation

Carbon nanotubes (CNTs) with different degree of surface oxidation were used as supports for 5wt.% Co catalysts. CNTs and Co/CNT catalysts were analyzed by XPS, nitrogen adsorption, TEM and electron diffraction to reveal their structure. High oxidation degree of CNT surface (8.6at.% of O) and low Co loading led to predominantly amorphous Co species. This resulted in the absence of catalytic activity in both CO and CO2 hydrogenation in opposite to the catalyst supported on less oxidized CNTs (5.4at.% of O) where Co species were found to be crystalline. Thermal treatment of inactive catalyst in H2 and He led to the formation of Co crystal phase which was active in catalysis. Co particle size in catalyst supported on strongly oxidized CNTs was unchanged during CO hydrogenation in opposite to Co supported on less oxidized CNTs. Carbon shell formation on the surface of amorphous Co particles during CO hydrogenation was revealed, which testified CO disproportionation. Qualitative mechanism of CO hydrogenation on small Co particles was proposed.

Isotopic Apportioning of Hydrogen/Deuterium on the Surface of an Activated Iron Carbide Catalyst

Fischer–Tropsch (FT) synthesis continues to receive widespread attention. Even after 90 years of investigation, the mechanistic route has yet to be fully defined. FT, as a polymerization process, uses CO and H2 as the reactants to produce a broad spectrum of hydrocarbons. Since the conception of the FT synthesis, several different isotopic routes have been employed for mechanistic studies. Various isotopes, such as 13C, 14C, 18O, and 2H, have been utilized through different types of experiments to shed light on the active site(s), the rate-limiting step, and the catalytic pathways. Direct evidence in the FT mechanism has been uncovered by utilizing experiments such as H2/D2 switching trials, as these experiments attempt to shed light on the rate-limiting step of CO hydrogenation. However, before hydrogen participates in the mechanism of CO hydrogenation, it may first dissociatively adsorb on the catalyst surface. The aim of this work is to ascertain to what extent H and D partition on the surface. This is accomplished by passing an equimolar H/D gas mixture over the activated FT catalyst, followed by a TPD method to determine if the active carbide surface displays a partitioning preference toward one of the isotopes. If a preference is observed, then the interpretation of kinetic isotopic effect (KIE) data ascertained in the CO hydrogenation switching experiments could potentially be affected. However, only a very slight isotopic preference toward deuterium was observed, and it is deemed not significant enough to affect an interpretation of the KIE based on H/D switching.

CO hydrogenation reaction over Fe[sbnd]Co catalyst; a micro-kinetic approach for studying the kinetics and mechanism of hydrocarbon formation

The study addresses an enhanced approach for study of kinetics and mechanism of CO hydrogenation over FeCo catalyst. Kinetic models for rate of methane, paraffin and olefin formation have been developed by LHHW approach and information that obtained from UBI_QEP calculations.

Infrared spectroscopic study on the reaction mechanism of CO hydrogenation over Pd/CeO2

In situ infrared spectroscopy was applied to elucidate the reaction mechanism of CO hydrogenation over Pd/CeO2. Instead of direct dissociation of CO, a new reaction pathway is proposed for methane formation, involving geminal dicarbonyl intermediates and (HCO)2(a) intermediates, which may be located on the surface of Pd covered with thin layers of reduced ceria (SMSI effect). Transformation of methane formation sites into methanol formation ones by the oxidation with water vapor formed during the CO−H2 reaction is proposed, which may be located on the Pd (111) planes adjacent to ceria support.

Mechanism and Kinetics of the Catalytic CO−H2 Reaction: An Approach by Chemical Transients and Surface Relaxation Spectroscopy

The aim of this paper is to demonstrate the importance of providing time-resolved information in catalysis research. Two truly in situ methods will be presented and compared for their merits and drawbacks: chemical transient kinetics (CTK) and pulsed field desorption mass spectrometry (PFDMS). The presentation will be given by way of example choosing the syngas (CO/H2) reaction over cobalt-based catalysts as a catalytic process. Despite numerous efforts in the past, the mechanism of this reaction is still under debate. In CTK the reaction is studied on a metal-supported catalyst under flow conditions in a pressure range extending from atmospheric pressure down to 100 Pa. Sudden changes in the partial pressures of the reactants then allow following the relaxation to either steady-state conditions ("transients") or cleanoff ("back transients"). In PFDMS short field pulses of several volts per nanometer are applied to a model catalyst which resembles a single metal particle grain (a "tip"). These pulses intervene during the ongoing reaction under flow conditions at pressures ranging from 10(-1) to 10(-5) Pa and cause field desorption of adsorbed species. This method is particularly suited to detect reaction intermediates in a time-dependent manner since the repetition frequency of the pulses can be systematically varied. It is shown that both methods lead to complementary results. While CTK allows conclusions on the mechanism of CO hydrogenation by following the time-dependent formation of hydrocarbon species, PFDMS provides insight into the initial steps leading to adsorbed CxHy species. A quantitative assessment of the CTK data allows the demonstration that the catalyst under working conditions is in an oxidized rather than metallic state. The initial steps to oxidation are also traced by PFDMS. Most importantly, however, CTK results allow formulation of a reaction mechanism that is common for both hydrocarbon and oxygenate formation and is based on the occurrence of a formate-type species as the most abundant surface intermediate.

An in Situ FT-IR Study of CO Hydrogenation over Cerium Oxide

The CO hydrogenation over cerium oxide was studied by using a closed circulating reaction system and in situ Fourier transform infrared spectroscopy (FT-IR) at 300-673 K. The major products of the CO-H2 reaction over cerium oxide were found to be olefins (C=2-C=4), CH4, CO2, and H2O. IR bands due to surface oxygen-containing species such as formate, dioxymethylene, methoxy, and formyl were always detected during the CO-H2 reaction at 300-573 K over cerium oxide. No direct relationship between the surface oxygen-containing species and the production of hydrocarbons were found. The mechanism of CO hydrogenation on cerium oxide involving the surface redox was discussed.

Probe molecule studies of CO hydrogenation over Ru/SiO 2

The effects of the addition of small amounts of CH 3NO 2 to the reactants during CO hydrogenation were investigated under various reaction conditions. The major changes in the product distribution of CO hydrogenation on Ru/SiO 2 caused by CH 3NO 2 addition to the reactants included substantial reduction of CH 4 production, increased production of hydrocarbons in the C3 to C6 carbon fractions, and enhanced selectivity toward linear olefinic products. A substantial incorporation of carbon originating from the added CH 3NO 2 into these hydrocarbon products was observed. Changes in the product distribution took place without significant disturbance of the main reaction pathways leading to hydrocarbon formation, offering great potential for the use of CH 3NO 2 as a probe molecule in the study of the mechanism of CO hydrogenation.

Quantum chemical study on the mechanism of CO hydrogenation to oxygenates over vanadium-promoted rhodium catalyst

Extended Hückel calculations of CO hydrogenation to oxygenates have been carried out for vanadium-promoted rhodium catalyst. The results of these calculations show that the barrier for CH 3 migration to Rh-CO to form Rh-COCH 3 species is decreased in the case of vanadium-promoted catalyst. A discussion attempts to explain the role of the promoters.

Kinetics and mechanism of CO hydrogenation over Ni catalysts supported on various oxides

The mechanism of CO hydrogenation on Ni catalysts supported on various oxides has been established. Synthesis of methane homologs starts with the condensation of two surface species of either CHOH or CH2. Kinetic equations agree with the observed behavior.

CO hydrogenation using cobalt/manganese oxide catalysts. Comments on the mechanism of carbon–carbon bond formation

A detailed investigation of the mechanism of CO hydrogenation over cobalt/manganese oxide catalyst is described. In particular, the reaction of a range of oxygenated compounds (methanol, ethanol, ethanol, propan-1-ol, propan-2-ol and propanl), diazo compounds (diazoacetaldehyde and ethyldiazoacetate ) is discussed in addition to the use of low concentrations of dioxygen as a molecular probe during CO hydrogenation. The results of this study indicate that hydroformylation of a C2 surface intermediate cannot account for the high yields of propene and low yields of methane observed with this catalyst. Based on this study a reaction mechanism for carbon–carbon bond formation in the Fischer–Tropsch reaction is proposed involving α- hydroxylated metal-alkyl as an important intermediate, the formation of which involves the coupling of a number of electrophilic and nucleophilic C1 surface intermediates.

EHMO calculations for the reaction of CO/H 2 on Ru/SiO 2 and Pt/SiO 2 catalysts

EHMO calculations of the chemisorption of CO, of the co-adsorption of H 2 and CO, and of the bonding of the intermediate species -CH 2, -CH 3 and -CH 2-CH 3 have been carried out for Ru/SiO 2and Pt/SiO 2 catalysts. Some experimental results for these catalysts have been elucidated theoretically on the basis of the carbidic mechanism of CO hydrogenation to yield hydrocarbons.