CO Turnover Research Articles

An unexpected and robust nickel (II)-based ether-bonded homogeneous system: Effective for photocatalytic CO2-to-CO transformation in visible light

Three halogen-containing pyridine carboxylic acid ligands (6-fluoropicolinic acid, 6-chloropicolinic acid, and 6-bromopicolinic acid) were combined with nickel to form three types of complexes: Ni-O ((6,6′-oxydipicoline acid) Ni(H2O)2), Ni-(OH)2((6,6′-hydroxydipicoline acid) Ni(H2O)2) and Ni-Br2((6,6′-bromodipicoline acid) Ni(H2O)2). Interestingly, the nickel salt reacted with 6-fluoropicolinic acid to form the ether-type complex Ni-O, with 6-chloropicolinic acid to form the hydroxy-type complex Ni-(OH)2, and with 6-bromopicolinic acid to form a Ni-Br2 complex with unchanged ligands. The Ni-O and Ni-(OH)2 complexes are novel in structure and especially the Ni-O show an excellent catalytic performance on reduction of CO2 to CO. After 12 h of irradiation, the Ni-O complexes attained a CO turnover number (TON) of 5664.3 and 86.7 % selectivity. The configurations of the complex catalysts have been demonstrated to play a major role in photocatalysis, by integrating analysis of high resolution mass spectrometry (HRMS) as well as density-functional theory (DFT). A potential proton-coupled electron transfer (PCET) mechanism of two-stage photocatalytic process is proposed.

Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO2 Reduction.

Three novel rhenium N-heterocyclic carbene complexes, [Re]-NHC-1-3 ([Re] = fac-Re(CO)3Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re-NHC-1 and Re-NHC-2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re-NHC-2 differs from Re-NHC-1 by replacing N-H with an N-benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re-NHC-2 with the larger pyrene gives Re-NHC-3. The two-electron electrochemical reductions of Re-NHC-2 and Re-NHC-3 result in the formation of the five-coordinate anions that are capable of electrocatalytic CO2 reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re-Re bound dimer intermediates at the second cathodic wave R2. All three Re-NHC-1-3 complexes are active photocatalysts for the transformation of CO2 to CO, with the most photostable complex, Re-NHC-3, being the most effective for this conversion. Re-NHC-1 and Re-NHC-2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re-NHC-3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re-NHC-3 is red-shifted compared to those of Re-NHC-1 and Re-NHC-2, and previously reported similar [Re]-NHC complexes. This observation, together with TD-DFT calculations, suggests that the nature of the lowest-energy optical excitation for Re-NHC-3 has π→π*(NHC-pyrene) and dπ(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re-NHC-3 are attributed to the extended conjugation of the π-electron system, leading to the beneficial modulation of the strongly electron-donating tendency of the NHC group.

Homogeneous–Heterogeneous Hybrid Artificial Photosynthesis Induced by Organic Semiconductors with Controlled Surface Architectures

AbstractPhotocatalysis is considered an effective approach for converting CO2 into high‐value‐added chemicals. However, practical implementation of this technology is limited by the efficiency and stability of photocatalysis. Herein, an interfacial control strategy is proposed to optimize the homogeneous‐heterogeneous hybrid photocatalysis by enhancing the interaction between light‐harvesting semiconductors (LHS) and molecular active centers (MAC). Based on this strategy, self‐assembled organic semiconductors with controlled surface architectures are constructed using 1,6‐bis(phenylethynyl)pyrene building blocks to act as LHS. Combining with the classical MAC, an excellent CO2 photoreduction performance is achieved with a CO turnover number of > 2980 maintaining long‐term stability with a selectivity of > 90%, and an apparent quantum yield of > 2.3%. Theoretical calculations combined with in situ and transient spectroscopy studies reveal that the optimized biphase interface dominates the synergy between the homogeneous and heterogeneous photocatalysts. This strategy and the proposed mechanism of interactions will contribute to the design of future artificial photosynthesis systems.

Polyaniline anchoring environment facilitates highly efficient CO2 electroreduction of cobalt phthalocyanine over a wide potential window

Metallophthalocyanines (MPcs) with the especially active metal-N4 catalytic sites have the great potential for the electrocatalytic CO2-to-CO conversion. However, the masking of metal-N4 catalytic sites attributed to the strong π-π interaction and the difficulty in capturing CO2 severely limits their catalytic efficiency. Herein, we designed a self-supported electrode that tetra-β-carboxy phthalocyanine cobalt(II) (TcPcCo) was anchored into polyaniline (PANI) chain in isolation using the carbon paper (CP) as a support. The PANI with unique porous structure provides large surface area, rapid electron transfer, and unimpeded pathways for CO2 diffusion. More importantly, sufficient CO2 captured by PANI can continuously spill over to metal-N4 active sites and be reduced into CO. The synthesized electrode for CO2 reduction reaction (CO2RR) could maintain a high CO Faraday efficiency of over 95% (maximum 99.3%) at an extraordinary wide potential range of 450 mV (from -0.50 V to -0.95 V vs. RHE), ranking foremost among the previously reported phthalocyanine-based electrocatalysts. Furthermore, relative to TcPcCo, the CO turnover frequency (TOF) of PANI-TcPcCo/CP significantly improves, especially at the more negative operating potential (about 7 ∼ 68 times). Theoretical calculations further reveal that the introduce of PANI optimizes the free energy barrier of CO2RR intermediate at Co-N4 site, thus accommodating the*COOH formation, and also suppresses the competition of hydrogen evolution reaction. This work presents a new perspective on advancing the catalytic CO2RR capability of molecular materials by regulating the chemical environment surrounding a catalytically active site.

Mediating heterogenized nickel phthalocyanine into isolated Ni-N3 moiety for improving activity and stability of electrocatalytic CO2 reduction

The poor stability of heterogenized nickel phthalocyanine (NiPc) has hindered its application as a desirable catalyst for electrocatalytic carbon dioxide reduction. Herein, the electrocatalytic stability of heterogenized NiPc on nitrogen-doped hollow carbon nanospheres (NiPc@NHCSs) can be optimized via mediating NiPc into Ni-NC moiety (Ni-NC/NHCSs-Y) to construct a stable Ni catalytic unit. Different from the poor activity of NiPc@NHCSs (CO Faradaic efficiency, FECO< 80%, stability < 1200 s), the optimal catalyst (Ni-NC/NHCSs-600) with unsaturated Ni-N3 nitrogen-vacancy structure, displays FECO of 98.57% and CO turnover frequency of 3.75 s−1 at − 0.87 V vs. RHE, and stable operation over 14 h (−0.82 V vs. RHE). The stabilization mechanism and the temperature effect on the structure-activity relationship are systematically explored, which successfully steers the design of stable Ni-NC catalytic unit on different carbon substrate, while a zinc-CO2 rechargeable battery is constructed, displaying a peak power density of 0.64 mW cm−2 and FECO of 91.45%.

A promising single-atom Co-N-C catalyst for efficient CO2 electroreduction and high-current solar conversion of CO2 to CO

Excavating highly efficient M-N-C electrocatalysts for electrochemical CO2 reduction (ECR) is of paramount importance. Herein, we report a single-atom Co-N-C catalyst (CoN4-CB) with a high CO Faradaic efficiency (FECO, 98.7%) in ECR comparable to that of a similarly prepared NiN4-CB catalyst. Impressively, CoN4-CB demonstrates a CO turnover frequency of 27173 h−1 and a CO current density of −33.6 mA∙cm−2 at −0.76 V, 20.2 and 6.8 times higher than that of NiN4-CB, respectively. In a solar-driven ECR system composed of a Si solar cell and a flow cell, CoN4-CB shows a remarkably large current density of 98.3 mA∙cm−2 with an average FECO of 92.1%. Theoretical calculations suggest that the energy barrier for *COOH formation largely decreases on CoN4 sites compared with NiN4 sites, leading to a low onset potential and high activity for CO production. This work will boost the development of efficient M-N-C electrocatalysts and further practical application of solar-driven ECR system.

Cu–Mn–O nano-particle/nano-sheet spinel-type materials as catalysts in methanol steam reforming (MSR) and preferential oxidation (PROX) reaction for purified hydrogen production

Nanocomposite Cu–Mn–O nano-particle (CuMnNP) and nano-sheet (CuMnNS) catalyst were successfully prepared using a one-step hydrothermal method in the absence of any templating reagent. Materials were characterised applying various structural techniques. SEM images showed that composite Cu–Mn oxide sheets were tailor-made synthesised by a one-pot urea-abetted protocol. Conversely, upon replacing carbamate by Na2CO3, oxidised metal Cu–Mn particles could be obtained. The formation of bulk mixed Cu–Mn phases resulted in an enhanced crystal lattice oxygen reactivity in CuMnNS. XPS, XRD and TPR measurements confirmed the presence of the Cu+ and Cu2+ species in nano-catalysts, and CuMnNS nanomaterials possessed more surface defects, thus causing a higher O2 adsorption/storage capacity. CuMnNS presented a superior catalytic activity as opposed to CuMnNP in the preferential oxidation (PROX) pathway of CO. With both CO2 and H2O in feed, a decrease in CO turnover was observed, due to a competitive interface binding of CO, CO2 and H2O. Compared to CuMnNP, CuMnNS also demonstrated a high time-on-stream conversion of methanol for the reforming for all operating conditions.

Typical transition metal single-atom catalysts with a metal-pyridine N structure for efficient CO2 electroreduction

It is of great importance to establish a definite relationship between structure and catalytic properties for developing highly efficient single-atom catalysts (SACs); however, this remains a challenge. Herein, three single atoms anchored on a nitrogen-doped carbon matrix (M-N-C, M = Fe, Co, Ni) with a metal-pyridine N structure were prepared and investigated as catalysts for electrochemical CO2 reduction. M-N-C exhibit promising capability for CO2-to-CO conversion with the selectivity order of Ni > Co > Fe and the activity order of Co > Ni > Fe, in contrast to the previously reported performance order of Ni > Fe > Co. Among M-N-C catalysts, Ni-N-C shows superior selectivity (approximately 100% CO Faradaic efficiency from −0.66 V to −0.96 V), whereas Co-N-C exhibits high activity (CO current density = −24.24 mA cm−2 and CO turnover frequency = 7182 h−1 at −0.86 V) and capability of yielding syngas (CO/H2 = 0.5∼2.11). Density functional theory calculations support the experimental selectivity and activity orders. This study presents a new possibility for improving the catalytic performance of SACs by investigating the coordinated environments of metal atoms.

Carbon monoxide powered fuel cell towards H2-onboard purification

Proton exchange membrane fuel cells (PEMFCs) suffer extreme CO poisoning even at PPM level (<10 ppm), owning to the preferential CO adsorption and the consequential blockage of the catalyst surface. Herein, however, we report that CO itself can become an easily convertible fuel in PEMFC using atomically dispersed Rh catalysts (Rh-N-C). With CO to CO2 conversion initiates at 0 V, pure CO powered fuel cell attains unprecedented power density at 236 mW cm−2, with maximum CO turnover frequency (64.65 s−1, 363 K) far exceeding any chemical or electrochemical catalysts reported. Moreover, this feature enables efficient CO selective removal from H2 gas stream through the PEMFC technique, with CO concentration reduced by one order of magnitude through running only one single cell, while simultaneously harvesting electricity. We attribute such catalytic behavior to the weak CO adsorption and the co-activation of H2O due to the interplay between two adjacent Rh sites.

Efficient Electrochemical Reduction of CO2 by Ni–N Catalysts with Tunable Performance

Developing highly selective and durable catalysts for electrochemical CO2 reduction (ECR) to valuable chemicals is essential for curbing CO2 emissions into the atmosphere and to close the anthropogenic carbon cycle. Herein, we report facile and scalable synthesis of Ni/NC as an efficient low-cost catalyst for ECR to CO. The electrocatalytic properties of the catalysts can be readily tailored by tuning N configurations and the fraction of single Ni sites in Ni/NC via manipulation of the N precursor and annealing conditions. The Ni/NC catalyst designed by thermal treatment of a composite mixture of carbon black, Ni(II) acetate, and polyacrylonitrile achieved low overpotential CO2 to CO conversion with a Faradaic efficiency (FE) as high as 96.5% at −0.9 V (vs reversible hydrogen electrode, RHE), a current density reaching −12.6 mA cm–2 at −1.2 V (vs RHE) corresponding to a CO turnover frequency of up to 4760 h–1. In addition, high FEs for CO formation (≥84.7%) were obtained in a wide potential range from −0.8 to −1.2 V (vs RHE) on the Ni/NC electrode. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra in combination with multiple electrochemical results suggested that the pyrrolic N and Ni–N structures concertedly play a determinant role in the catalytic performance of Ni/NC during the CO2 reduction reaction.

Effects of Ag promotion and preparation method on cobalt Fischer-Tropsch catalysts supported on silica-modified alumina

A series of silver-promoted, 20 wt% cobalt Fischer-Tropsch synthesis (FTS) catalysts supported on an alumina modified with 5 wt% silica were prepared using two methods: traditional incipient wetness impregnation (IWI) and a new solvent deficient precipitation (SDP) technique. Catalysts containing silver promoter concentrations of 0.3, 0.6, 1.2, and 2.5 wt% were prepared using each of the two methods. Silver improved the reducibility of the cobalt significantly, lowering reduction temperatures by up to 100 °C, and increasing the extent of reduction by up to 35%. Further, in both preparation methods, changing the silver loading altered the cobalt dispersion. The smallest Co crystallite size was achieved with 1.2 wt% Ag loading, which produced average cobalt crystallite sizes of about 7 nm. The Fischer-Tropsch CO consumption rate increased with decreasing crystallite size and thus was highest for 1.2 wt% Ag. Intrinsic CO consumption rates per Co site (CO turnover frequency) were also measured for each catalyst. A clear increase in the intrinsic turnover frequency (TOF) was observed as Ag loading was increased from 0.6% to 1.2 wt% for both preparation methods. Both 1.2 wt% Ag catalysts produced TOF’s of ∼0.050 s−1 (equivalent to a CO consumption rate of ∼65 mmol gcat−1 h−1), which are 20–30% higher than the catalysts containing 0.3, 0.6, and 2.5 wt% Ag and are comparable to reported rates for commercial FTS catalysts. Higher loadings of Ag (2.5 wt%) resulted in higher extents of reduction, but led to lower TOFs and larger Co crystallite diameters, which is assumed to be due to blockage of active sites and changes in the Ag-Ag and Ag-Co coordination ratio. Thus, silver promotion appears to improve catalytic performance by enhancing cobalt reduction, dispersion, and electronic properties. The SDP and IWI methods produced catalysts with essentially the same properties, but the SDP method is a simpler, one-pot technique that offers many potential advantages.

Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2

Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–Nx moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–Nx moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–Nx moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.

PROX reaction of CO in H2/H2O/CO2 Water–Gas Shift (WGS) feedstocks over Cu–Mn/Al2O3 and Cu–Ni/Al2O3 catalysts for fuel cell applications

Abstract Catalytic conversion performance of the γ-Al2O3-supported Cu-, Mn- and Ni-based materials for the selective preferential oxidation of CO was investigated. The influence of CO2 and H2O in the reactor feed gas on activity and selectivity was lastly investigated for copper bimetallic catalysts containing manganese and nickel. Cu/Al2O3 proved to be optimal among the monometallic examined. It was observed that even in the absence of H2, CO was effectively converted. Upon H2 presence, a fraction of oxygen is utilized for the latter, which results in the decrease of CO turnover. Cu–Mn/Al2O3 surpassed other single and mixed metal oxide catalysts which could be due to a combination of surface desorption capacity and redox properties. The oxygen storage capacities (OSC) were in the order of Cu–Mn/Al2O3 > Cu–Ni/Al2O3 > Cu/Al2O3 > Mn/Al2O3 > Ni/Al2O3, in an agreement with reactivity. The efficiency at low process temperatures was enhanced by adding H2O vapors, nonetheless, decreased at high-end range due to the reverse water–gas shift (RWGS). Also, the rate of CO conversion was high utilizing Cu–Mn/Al2O3; furthermore, even upon applying H2O and CO2. CO2 notably affected CO conversion, and ultimately, decreased the activity, which, conversely, remained stable upon time-on-stream.

Insights in the Catalytic Product Selectivity Trends of Single Site Transition Metal-Nitrogen-Doped Carbons during the Electrochemical Reduction of CO2 to CO

The direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted much attention in part due to the fundamental surface catalytic challenges related to activity and selectivity, in part owing to its potential practical importance in CO2 valorization for energy and chemicals application.1 In particular, the selective 2-electron reduction to CO has captivated chemists’ and engineers’ minds for the design of technological CO2-consuming cathodes (CCCs) for use as counter electrodes in the polymer-chloralkaline industries, where fossil-based CO currently still serve as feedstock for production of e.g. polycarbonate and polyurethanes. Work on solid catalysts for the selective electrochemical reduction of CO2 to CO has been limited to the traditional noble metal catalysts such as Ag and Au.2 Here, we present new advances in our understanding of the CO2 reduction process on N-coordinated, non-noble metal-doped nano-porous carbons (M-N-C, M=Mn, Fe, Co, Ni, Cu). These materials are single site catalysts comprising active M-Nx moieties. We investigate the trends in their intrinsic electrochemical reactivity, CO turnover frequencies, and CO efficiencies and show that Fe-N-C and Ni-N-C catalysts outperform Au or Ag. We model the active site by Density Functional Theory (DFT) and arrive at a consistent atomic-scale understanding of their potential-dependent CO and hydrocarbon selectivity. Linking DFT predictions directly with experiments we explain why Fe-N-C and Ni-N-C represent the most cost-effective and energy-efficient carbon-based catalysts for future CO generation inside CCCs.

Effect of Carbon Porosity and Cobalt Particle Size on the Catalytic Performance of Carbon Supported Cobalt Fischer–Tropsch Catalysts

Cobalt catalysts supported on activated carbon and on carbon nanotubes (CNTs) with different porosities were prepared by an incipient wetness impregnation method and characterized by a series of methods. The samples were reduced and then evaluated in a fixed-bed reactor for Fischer–Tropsch (FT) synthesis. The porosity of the carbon support greatly influenced the microstructure, the reducibility, the dispersion, and the FT performance of the cobalt catalysts. The carbon structure and the cobalt dispersion determined CO conversion. CNTs with larger pore sizes were more stable at high temperature in a H2 atmosphere. The cobalt particle size impacted the CO turnover frequency (TOF) and the C5+ selectivity. Larger cobalt particles (up to 7 nm) resulted in higher TOF and C5+ selectivity; for cobalt particles larger than 7 nm, no such increase in these parameters was seen. The carbon support influenced the C5+ selectivity and the C5+ hydrocarbon distribution. Interestingly, the olefin/paraffin ratio of C2 was lo...

Incorporation of Highly Dispersed Cobalt Nanoparticles into the Ordered Mesoporous Carbon for CO Hydrogenation

Highly dispersed cobalt nanoparticles were incorporated into the ordered mesoporous carbon (OMC) via the ultrasonically assisted vacuum-pressure impregnation (UAVI) for CO hydrogenation. The as-synthesized samples were subjected to the characterization of N2 physisorption, HAADF-STEM, XRD, XPS, magnetic measurement, H2 chemisorption and 59Co zero field spin-echo NMR. It is found that the cobalt particles are exclusively distributed into the channels of OMC and the rigid frameworks significantly bridle the growth of active species to obtain ultrafine cobalt particles with size less than 4 nm. The highly dispersed cobalt nanoparticles perform superior catalytic performances during the CO hydrogenation as compared to the catalyst prepared by the conventional impregnation. The observed stability of novel catalysts outperforms the conventional one during the 96 h evaluation on stream due to the confinement effect to the sintering and conglomeration of active species. On the other hand, a typical structural sensitivity is also observed for the highly dispersed catalysts and the coordinative-unsaturated disordered stacking of cobalt atoms renders higher efficiency to improve CO turnover frequency. Besides, a high selectivity to mixed alcohols is also obtained for the highly dispersed catalysts and the yields of methane and methanol decrease gradually with increasing the particle size. Unfortunately, the highly dispersed particles also display a relatively high selectivity to CO2. Highly dispersed cobalt nanoparticles with size less than 4 nm were exclusively incorporated into the ordered mesoporous carbon by ultrasonically assisted vacuum-pressure impregnation. The high-surface-area and chemical inertness of carbonaceous substrates enable the ultrafine particles to outperform during CO hydrogenation.

Effect of carbon support on Fischer–Tropsch synthesis activity and product distribution over Co-based catalysts

The ordered mesoporous carbon (CMK-3) was prepared using SBA-15 as template and sucrose as carbon source. Co catalysts supported on CNTs, CMK-3 and AC were prepared by incipient wetness. Their physical properties, morphologies and Fischer–Tropsch synthesis (FTS) activities were assessed. It was found that Co3O4 particles mainly dispersed inside the tubes (or pores) of Co/CNTs and Co/CMK-3 but outside the pores of Co/AC. Co3O4 particle size depended closely on the support structure. After H2 reduction, the three catalysts had a similar cobalt particle size. The cobalt particle size of Co/CNTs was maintained well due to the special confinement effect of CNTs. CNTs were more stable at high temperature in H2 atmosphere than CMK-3 and AC and the reduction degree of Co/CNTs was higher than the other two catalysts. The good crystallized graphitic structure facilitated the electron transfer between the cobalt and CO molecules on Co/CNTs. So Co/CNTs had a better FTS performance than Co/CMK-3 and Co/AC catalysts. Carbon support structure influenced the C5+ selectivity, C5+ hydrocarbon distribution and CO turnover frequency. Furthermore, a positive relationship existed between C2–C4 olefin/paraffin ratio and C5+ selectivity. High olefin content and low space velocity benefit the heavy hydrocarbon production.

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

Fischer–Tropsch (FT) synthesis, the conversion of CO and H2 to long-chain hydrocarbons, is performed at relatively low temperatures and high pressures. Density functional theory shows that the CO coverage on Co(0001) increases gradually until a (√3×√3)R30o–CO configuration (1/3ML) is formed. This structure is stable over a relatively wide temperature and pressure range near FT conditions, until a phase transition to a (2√3×2√3)R30o-7CO structure occurs at high CO pressures. The 1/3ML CO coverage reduces the H2 binding enthalpy from −121 to −74kJ/mol and reduces the hydrogen coverage to <0.3ML. The 1/3ML coverage reduces the stability of reaction intermediates by 10–30kJ/mol. For the CO insertion mechanism, where RCH* groups couple with CO, the reduced stabilities decrease the overall surface barrier from 175 to 111kJ/mol. The reduced barrier increases the CO turnover frequency to 0.02s−1, close to experimental values and five orders of magnitude higher than the corresponding low-coverage value.

Density Functional Theory Study of the CO Insertion Mechanism for Fischer−Tropsch Synthesis over Co Catalysts

Different mechanisms have been proposed for Fischer−Tropsch synthesis, the conversion of CO and H2 to long-chain alkanes. Density functional theory calculations indicate that CO activation has a barrier of 220 kJ/mol on Co(0001), and hence the concentration of surface C or CH2 species is likely too low to explain the high chain growth probability. Hydrogenation lowers the C−O dissociation barrier to 90 kJ/mol for HCO and to 68 kJ/mol for H2CO; however, CO hydrogenation has a high energy barrier of 146 kJ/mol and is +117 kJ/mol endothermic. We propose an alternative propagation cycle starting with CO insertion into surface RCH groups. The barrier for this step is 80 kJ/mol. RCHCO is subsequently hydrogenated to RCH2CHO, which undergoes C−O dissociation with a barrier of 50 kJ/mol. The hydrogenation barriers are 120 and 48 kJ/mol along the dominant reaction path. The calculated CO turnover frequency for the proposed CO insertion mechanism is 1 to 2 orders of magnitude faster the hydrogen-assisted CO activat...

Spatial Distribution of the CO Transient Response of a PEFC

We report on the investigation of the current distribution of a PEFC under the influence of CO impurities in the hydrogen feed stream by application of the segmented cell system. Stepping the CO partial pressure of the anode feed stream from 0 to 100 ppm CO the current response of the cell was recorded for different stoichiometric flows. A poisoning time delay occurs along the flow path due to the anode flow rate. Also anode and cathode processes, e.g. CO turnover and oxygen mass transport limitation contribute to the time response of the cell.