Mechanism of Cobalt-Catalyzed CO Hydrogenation: 1. Methanation

Facile CO bond cleavage on polynuclear vanadium nitride clusters V4N5. Phys Chem Chem Phys 2022;24:29765-29771. [PMID: 36458914 DOI: 10.1039/d2cp04304a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Identifying the structural configurations of precursors for CO dissociation is fundamentally interesting and industrially important in the fields of, e.g., Fischer-Tropsch synthesis. Herein, we demonstrated that CO could be dissociated on polynuclear vanadium nitride V₄N₅^- clusters at room temperature, and a key intermediate, with CO in a N-assisted tilted bridge coordination where the C-O bond ruptures easily, was discovered. The reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemistry calculations, and the nature of the adsorbed CO on product V₄N₅CO^- was further characterized by a collision-induced dissociation experiment. Theoretical analysis evidences that CO dissociation is predominantly governed by the low-coordinated V and N atoms on the (V₃N₄)VN^- cluster and the V₃N₄ moiety resembles a support. This finding strongly suggests that a novel mode for facile CO dissociation was identified in a gas-phase cluster study.

Bimetallic Pt-Co Catalysts for the Liquid-Phase WGS

Bimetallic Pt-Co catalysts derived from cobalt aluminate spinel were investigated in the liquid-phase water–gas shift (WGS) reaction and CO hydrogenation. Liquid-phase WGS is a key reaction in the aqueous-phase reforming (APR) of polyols; thus, WGS activity is essential to formulate good APR catalysts. In this work, catalysts with different Pt/Co molar ratios were synthesized together with a reference Pt/alumina. All the synthesized catalysts were characterized by various techniques in order to gain knowledge on their structural and surface characteristics. WGS activity was tested with a feedstream of CO/H2O = 1/15 (space-time of 76.8 kgcat·s/molCO), isothermal operation at 260 °C and 50 bar, for 10 TOS. Bimetallic Pt-Co catalysts showed improved activity in liquid-phase WGS in comparison to bare Co or Pt catalysts, which was ascribed to the synergistic effect. Despite being subjected to an increased hydrogen concentration in the feedstream (H2/CO between 0 and 12/3), these catalysts maintained a preferential selectivity towards WGS activity. In addition, the effect of temperature (220–260 °C) and pressure (25–50 bar) was investigated over a catalyst with 0.3Pt/CoAl. CO conversion and CO2 yield were more sensitive to temperature, while a higher pressure favored methane production. The measured activation energy in the 220–260 °C temperature range was 51.5 kJ/mol.

The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.

Effects of calcination temperatures on the structure-activity relationship of Ni-La/Al2O3 catalysts for syngas methanation

A series of Ni–La/Al₂O₃ catalysts for the syngas methanation reaction were prepared by a mechanochemical method and characterized by thermogravimetric analysis (TG-DTA), X-ray fluorescence (XRF), X-ray diffraction (XRD), N₂ adsorption–desorption, H₂ temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The calcination temperatures (350–700 °C) had significant impacts on the crystallite sizes and interactions between NiO and Al₂O₃. The catalyst calcined at 400 °C (cat-400) showed a 12.1% Ni dispersion degree and the maximum bound state of NiO (54%) through the Gaussian fitting of H₂-TPR. Cat-400 also achieved the highest CO conversion, CH₄ selectivity and yield. Cat-400 exhibited good stability and catalytic activity in a lifetime testing of 200 h. The deactivation of cat-400 was mainly caused by carbon deposition according to the data from XRD, TG-DTG and XPS.

Calcination temperature affects the existing types of NiO, and the influence of the three NiO types on the catalytic activity of samples is bound type ≫ free type > combined type.

The consequences of surface heterogeneity of cobalt nanoparticles on the kinetics of CO methanation

The CO hydrogenation reaction was studied under methanation conditions (H₂/CO >3, 250–300 °C) on Co/SiO₂ catalysts with different mean Co nanoparticle size (d_p = 4 nm, 13 nm and 33 nm).

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 CH _x intermediates, in line with decreased CH₄ selectivity and increased chain-growth rate.

Insight into the structure and morphology of Run clusters on Co(111) and Co(311) surfaces

The adsorption configurations, growth modes and morphology of a Ru promoter under the approximate conditions of cobalt catalyzed Fischer–Tropsch synthesis (FTS) were investigated by density functional theory (DFT) calculations.

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.