Cobalt Surface Research Articles

The Molecular Adsorption of Carbon Monoxide on Cobalt Surfaces: A Dft Study

The theoretical molecular adsorption energies, vibrational frequencies and total density of states of carbon monoxide (CO) on the (100), (110) and (111) surfaces of the face-centred cubic (FCC) crystalline phase of metallic cobalt were investigated using density functional theory calculations. The on-top adsorption state and three surface coverages were used for comparison of the results. The geometries of cobalt FCC surfaces, as well as those with adsorbed CO molecules and the CO binding energies were calculated with the generalised gradient approximation (GGA-D) using the revised revPBE-D3(BJ) functional. The theoretical results for adsorption energies of carbon monoxide were proportional to the electron density of the cobalt surfaces, according to the following order: FCC (100) > FCC (110) > FCC (111). For CO adsorbed on the surface of cobalt metal the C–O distance increases, producing a weakening of the bond and the calculated stretching frequency decreases when compared with the isolated molecule.

Density Functional Theory and Microkinetic Studies of Bio-oil Decomposition on a Cobalt Surface: Formic Acid as a Model Compound

Density functional theory calculations and microkinetic modeling were used to study the decomposition mechanisms of the bio-oil model compound formic acid over a cobalt-stepped surface. Zero-point-energy-corrected activation barriers and reaction energies and the rate and equilibrium constants of various elementary reactions were obtained. Formic acid dissociation likely starts from dehydrogenation and dehydroxylation, with activation barriers of less than 0.5 eV. The generation of an HCOO intermediate is thermodynamically favored, but such a compound is energetically difficult to convert. COOH formation is fast and dominant at low temperatures, and it is converted rapidly after 450 K. The most favorable formic acid decomposition pathway is HCOOH → COOH → CO. Its rate-determining step is CO–OH scission, with an activation barrier of 0.66 eV and strong exothermicity of −1.19 eV.

Metal-support interaction on cobalt based FT catalysts - a DFT study of model inverse catalysts.

It is challenging to isolate the effect of metal-support interactions on catalyst reaction performance. In order to overcome this problem, inverse catalysts can be prepared in the laboratory and characterized and tested at relevant conditions. Inverse catalysts are catalysts where the precursor to the catalytically active phase is bonded to a support-like ligand. We can then view the metal-support interaction as a ligand interaction with the support acting as a supra-molecular ligand. Importantly, laboratory studies have shown that these ligands are still present after reduction of the catalyst. By varying the quantity of these ligands present on the surface, insight into the positive effect SMSI have during a reaction is gained. Here, we present a theoretical study of mono-dentate alumina support based ligands, adsorbed on cobalt surfaces. We find that the presence of the ligand may significantly affect the morphology of a cobalt crystallite. With Fischer-Tropsch synthesis in mind, the CO dissociation is used as a probe reaction, with the ligand assisting the dissociation, making it feasible to dissociate CO on the dense fcc Co(111) surface. The nature of the interaction between the ligand and the probe molecule is characterized, showing that the support-like ligands' metal centre is directly interacting with the probe molecule.

Increased thermal stability of activated N2 adsorbed on K-promoted Ni{110}.

Industrial synthesis of ammonia takes place at high temperatures and pressures via the dissociative adsorption of molecular nitrogen on a transition metal catalyst. In contrast, biological ammonia synthesis occurs under ambient conditions via the hydrogenation of intact molecular nitrogen at the active site of an enzyme. We hypothesise that the latter process may be mimicked within an inorganic system if the intact nitrogen molecule can be polarised, rendering it particularly susceptible to attack by hydrogen. Furthermore, by analogy with the surface chemistry of carbon monoxide at alkali-modified nickel and cobalt surfaces, we consider whether such a polarisation may be achieved by coadsorption with potassium on the same or similar transition metals. Here, we report on reflection absorption infrared spectroscopy results, interpreted with the aid of first-principles density functional calculations, which reveal both similarities and differences between the behaviour of carbon monoxide and nitrogen. Importantly, our calculations suggest that the surface-induced dipole of molecular nitrogen can indeed be enhanced by the coadsorbed alkali metal.

Understanding FTS selectivity: the crucial role of surface hydrogen.

Monomeric forms of carbon play a central role in the synthesis of long chain hydrocarbons via the Fischer-Tropsch synthesis (FTS). We explored the chemistry of C1Hxad species on the close-packed surface of cobalt. Our findings on this simple model catalyst highlight the important role of surface hydrogen and vacant sites for product selectivity. We furthermore find that COad affects hydrogen in multiple ways. It limits the adsorption capacity for Had, lowers its adsorption energy and inhibits dissociative H2 adsorption. We discuss how these findings, extrapolated to pressures and temperatures used in applied FTS, can provide insights into the correlation between partial pressure of reactants and product selectivity. By combining the C1Hx stability differences found in the present work with literature reports of the reactivity of C1Hx species measured by steady state isotope transient kinetic analysis, we aim to shed light on the nature of the atomic carbon reservoir found in these studies.

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

Highly monodisperse cobalt crystallites, supported on Stöber silica spheres, as model catalysts for the Fischer-Tropsch synthesis were exposed to simulated high conversion environments in the presence and absence of CO utilising an in house developed in situ magnetometer. The catalyst comprising the smallest crystallites in the metallic state (average diameter of 3.2 nm) experienced pronounced oxidation whilst the ratio of H2O to H2 was increased stepwise to simulate CO conversions from 26% up to complete conversion. Direct exposure of this freshly reduced catalyst to a high conversion Fischer-Tropsch environment resulted in almost spontaneous oxidation of 40% of the metallic cobalt. In contrast, a model catalyst with cobalt crystallites of 5.3 nm only oxidised to a small extent even when exposed to a simulated conversion of over 99%. The largest cobalt crystallites were rather stable and only experienced measurable oxidation when subjected to H2O in the absence of H2. This size dependency of the stability is in qualitative accordance with reported thermodynamic calculations. However, the cobalt crystallites showed an unexpected low susceptibility to oxidation, i.e. only relatively high ratios of H2O to H2 partial pressure caused oxidation. Similar experiments in the presence of CO revealed the significance of the actual Fischer-Tropsch synthesis on the metallic surface as the dissociation of CO, an elementary step in the Fischer-Tropsch mechanism, was shown to be a prerequisite for oxidation. Direct oxidation of cobalt to CoO by H2O seems to be kinetically hindered. Thus, H2O may only be capable of indirect oxidation, i.e. high concentrations prevent the removal of adsorbed oxygen species on the cobalt surface leading to oxidation. However, a spontaneous direct oxidation of cobalt at the interface between the support and the crystallites by H2O forming presumably cobalt silicate type species was observed in the presence and absence of CO. The formation of these metal-support compounds is in accordance with conducted thermodynamic predictions. None of the extreme Fischer-Tropsch conditions initiated hydrothermal sintering. Seemingly, the formation of metal-support compounds stabilised the metallic crystallites and/or higher partial pressures of CO are required to increase the concentration of mobile, cobalt oxide-type species on the metallic surface.

Computational investigation of the kinetics and mechanism of the initial steps of the Fischer-Tropsch synthesis on cobalt.

A multi-site microkinetic model for the Fischer-Tropsch synthesis (FTS) reaction up to C2 products on a FCC cobalt catalyst surface is presented. This model utilizes a multi-faceted cobalt nanoparticle model for the catalyst, consisting of the two dominant cobalt surface facets Co(111) and Co(100), and a step site represented by the Co(211) surface. The kinetic parameters for the intermediates and transition states on these sites were obtained using plane-wave, periodic boundary condition density functional theory. Using direct DFT data as is, the microkinetic results disagree with the expected experimental results. Employing an exploratory approach, a small number of microkinetic model modifications were tested, which significantly improved correspondence to the expected experimental results. Using network flux and sensitivity analysis, an in-depth discussion is given on the relative reactivity of the various sites, CO activation mechanisms, the nature of the reactive chain growth monomer, the probable C2 formation mechanism, the active site ensemble interplay and the very important role of CO* surface coverage. The findings from the model scenarios are discussed with the aim of guiding future work in understanding the FTS mechanism and subsequent controlling kinetic parameters.

Sorption-enhanced hydrogen production by steam reforming of ethanol over mesoporous Co/CaO[sbnd]Al2O3 xerogel catalysts: Effect of Ca/Al molar ratio

A series of multifunctional Co/CaOAl2O3 catalysts (Co/XCA, X = 0, 0.25, 0.5, 0.75, and 1.0) with different Ca/Al molar ratio (X) were prepared for use in the sorption-enhanced hydrogen production by steam reforming of ethanol. Mesoporous structure was successfully formed in all the calcined catalysts, and surface area of the catalysts decreased with increasing Ca/Al molar ratio. Co3O4, CaCO3, and γ-Al2O3 phases were dominantly formed in the calcined catalysts. It was revealed that addition of Ca neutralized support acidity, resulting in a decrease of C2H4 selectivity. Addition of Ca also increased strong base sites which were related to sorption-enhanced hydrogen production at the initial reaction stage. In the XRD and H2-TPD analyses, Co/0.5CA catalyst retained the smallest crystallite size of metallic cobalt and the highest cobalt surface area among the catalysts. Catalytic activity in the steam reforming of ethanol exhibited a volcano-shaped trend with respect to Ca/Al molar ratio. Hydrogen yield increased with increasing cobalt surface area. Among the catalysts tested, Co/0.5CA catalyst with the highest cobalt surface area showed the best catalytic performance. Thus, cobalt surface area served as a crucial factor determining the catalytic activity for ethanol steam reforming. Low acidity and high surface area were also responsible for stable catalytic activity of Co/0.5CA catalyst in the steam reforming of ethanol.

Adsorption and Decomposition of Ethene and Propene on Co(0001): The Surface Chemistry of Fischer–Tropsch Chain Growth Intermediates

Experiments that provide insight into the elementary reaction steps of CxHy adsorbates are of crucial importance to better understand the chemistry of chain growth in Fischer–Tropsch synthesis (FTS). In the present study we use a combination of experimental and theoretical tools to explore the reactivity of C2Hx and C3Hx adsorbates derived from ethene and propene on the close-packed surface of cobalt. Adsorption studies show that both alkenes adsorb with a high sticking coefficient. Surface hydrogen does not affect the sticking coefficient but reduces the adsorption capacity of both ethene and propene by 50% and suppresses decomposition. On the other hand, even subsaturation quantities of COad strongly suppress alkene adsorption. Partial alkene dehydrogenation occurs at low surface temperature and predominantly yields acetylene and propyne. Ethylidyne and propylidyne can be formed as well, but only when the adsorbate coverage is high. Translated to FTS, the stable, hydrogen-lean adsorbates such as alkynes...

Roles of phosphorous-modified Al2O3 for an enhanced stability of Co/Al2O3 for CO hydrogenation to hydrocarbons

A phosphorous-modified γ-Al2O3 (P-Al2O3), where γ-Al2O3 support was prepared by sol-gel method with a high surface area of ∼350m2/g, has been applied for a preparation of cobalt-supported Co/P-Al2O3 catalysts. The Co/P-Al2O3 catalysts having a different P/Al molar ratio were investigated to elucidate the roles of phosphorous species on the γ-Al2O3 to the catalytic stability and product distribution for CO hydrogenation to hydrocarbons. The γ-Al2O3 surface was partially transformed to aluminum phosphates after phosphorous modification, and the newly formed aluminum phosphate phases simultaneously altered the surface hydrophilicity and cobalt dispersion as well. The partial formation of tridymite aluminum phosphate (AlPO4) phases on the P-Al2O3 support eventually enhanced the dispersion of the supported cobalt crystallites and suppressed the aggregation of cobalt nanoparticles by forming the strongly interacted cobalt crystallites on the P-Al2O3 surfaces. The phosphorous-modified Fischer-Tropsch synthesis (FTS) catalyst also significantly suppressed heavy hydrocarbon depositions due to an increased surface hydrophilicity originated from partially formed SiO2-like tridymite AlPO4 surfaces. A higher stability of the Co/P-Al2O3 catalyst at an optimal phosphorous content in the range of 0.5–1.0 mol% was attributed to homogeneously distributed cobalt crystallites and less deposition of heavy hydrocarbons by forming macro-emulsion droplets with the help of trace amount of alcohols formed during FTS reaction. This was confirmed by in-situ analysis of adsorbed intermediates with surface hydrophilicity and some surface characterizations such as crystallite size, reducibility, and electronic state of the supported cobalt nanoparticles.

Effect of incorporation manner of Zr on the Co/SBA-15 catalyst for the Fischer–Tropsch synthesis

The effect of incorporation manner of Zr on the textural properties, surface physicochemical properties, reduction degree and catalytic performance of Co/SBA-15 catalyst for the Fischer–Tropsch synthesis (FTS) was investigated. The catalysts were prepared by sequential impregnation, co-impregnation and in-situ synthesis method, respectively. The loading amount of Co and Zr remained constant in all catalysts with 15wt.% and 5wt.%, respectively. The catalysts were characterized by N2 adsorption–desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), temperature-programmed reduction (H2-TPR) and evaluated for the FTS. It was revealed that the Co3O4 was considered as the main surface cobalt phase, and the BET surface area and pore size were decreased regardless of the incorporation manner. The reduction degree of cobalt species increased from 80% (Co/SBA-15) to 90% (Co/Zr/SBA-15) and 85% (CoZr/SBA-15) with the addition of Zr, except for the Co/Zr-SBA-15 catalyst (74%). The catalyst prepared by sequential impregnation method (Co/Zr/SBA-15) showed the highest selectivity of long-chain hydrocarbons (C12–C22, 53%) with the chain growth probability α up to 0.84, which could be attributed to the highest reducibility of cobalt species.

Monte Carlo Simulation of Near- and Supercritical Hexane Fluid and Physisorption Phase Behavior

Monte Carlo (MC) simulations were used to determine the near- and supercritical fluid phase behavior of hexane and the adsorption thermodynamics of hexane on cobalt. Canonical ensemble MC simulations were used to compute the equation of state and chemical potentials, and Gibbs ensemble MC simulations (GEMC) were used to compute fluid phase coexistence. Overall, MC results with the TraPPE-UA force field were successful in representing near- and supercritical hexane thermodynamic behavior, and represent an improvement over treatments with a classical cubic equation of state. In addition, GEMC simulations predict a critical point for TraPPE-UA hexane of 509 K, 35 bar, and 0.22 g/mL, in excellent agreement with critical constants for real hexane. With regard to physisorption, hexane physisorption onto a (0001) cobalt surface at 523.15 K was modeled using grand canonical Monte Carlo (GCMC). GCMC excess adsorption results show crossover from adsorption to depletion around a bulk density of 0.1 g/mL and a global...

Structure Deformation and Electronic Spin States Modification in the Surfaces of Charged and Discharged Lithium-Ion Battery Cathodes

Structures and electronic spin states on active-surfaces for Li-ion (de) intercalation of Lithium Cobalt Oxides (LCO) are investigated by first principle calculation and molecular orbital analysis focusing on differences between charged and discharged states. Instability of the cathodes prevents realizing higher voltage charge of Li-ion Batteries (LIB). Especially, the degradation of the edge surfaces where Li-ions propagate in the charging and discharging processes decreases the capacity of the batteries severely. Despite of the long history of research and development of LIB, the degradation of surfaces of the cathodes has not been known well from electronic states point of view. In this study, by using electronic state calculations and orbitals analyses, we approach the mechanism of the surface deformation, focusing especially on the most probable edge surface of LCO. First, we established the edge surface model in the discharged states and performed the structure optimization with evaluation of the surface formation energy. Second, we deleted three quarters of Li-ions from the surface model and optimized the structure again. Comparison of the both structures shows that Li-ion de-insertion gives the flexibility of the coordination structures of the surface cobalt and promotes the surface structure distortion. Third, the electronic states of the both charged and discharged states were examined. We developed an analysis method of Kohn-sham orbitals which enable us to know the symmetry of the orbitals for valence band minimum and other bands, indicating that we can figure out the surface conditions from the ligand field theory for the surface cobalt and oxygen atoms. Our analyses show that the d-electrons of the surface cobalt atoms in the charged state occupy more unstable orbitals than those in the discharged states. Finally, we concluded that lithium de-intercalation causes surface deformation with unstable cobalt spin states and considered that the deformation brings about further structure degradations. We believe that the results can be applied to the other cathode materials containing transition-metal atoms.

Initial nitride formation during plasma-nitridation of cobalt surfaces

Nitridation of metal surfaces is of central importance in microelectronics and spintronics due to the excellent mechanical, thermal, and electrical properties of refractory nitrides. Here, we examine the chemical and structural modification of cobalt surfaces upon nitrogen plasma treatment, using in situ spectroscopic methods, as a method for synthesis of cobalt nitride thin films. We find that nitrogen is incorporated below the surface and forms an ultrathin film of CoN at temperatures as low as 50 °C. In addition, we observe the incorporation of oxygen and NO+ within the surface region. The nitrided cobalt surfaces are fully passivated by N, O, and NO+. These results provide a route for incorporation of cobalt nitride into a wide range applications.

TiO2 polymorph dependent SMSI effect in Co-Ru/TiO2 catalysts and its relevance to Fischer-Tropsch synthesis

Pure anatase and rutile TiO2 samples were synthesized by thermal treatment of reverse microemulsions and applied as supports for preparing Ru-promoted Co catalysts (0.5wt% Ru, 10wt% Co). The catalysts were characterized by ICP-OES, XRD, Raman spectroscopy, N2 physisorption, H2-TPR, electron microscopy (FESEM, HAADF-STEM), H2 chemisorption, XPS, and in situ IR-CO after H2 reduction and reaction with syngas, and their catalytic performance for Fischer-Tropsch synthesis (FTS) studied at industrial conditions (220°C, 2.0MPa, H2/CO=2). The two catalysts exhibited comparable mean Co particle sizes (5–6nm) as well as high and alike degrees of cobalt reduction (ca. 90%). The SMSI decoration effect arising during H2 reduction was much more pronounced for the anatase-supported catalyst resulting in lower cobalt-time-yield (CTY) compared to that supported on TiO2-rutile. In situ IR-CO under syngas conversion conditions showed equivalent cobalt surface reconstruction and nature of the surface Co0 sites for both catalysts in their working state, and revealed a partial reversibility of the SMSI effect during FTS by which a significant fraction of the decorated Co0 centers in the anatase-based catalyst was uncovered and became available for reaction. The implication of this effect on TOFs is discussed. The C5+ selectivity was higher for the rutile-based catalyst, although a clear impact of the SMSI effect on selectivities was not inferred from our results.

Molecular deposition of a macrocyclic cobalt catalyst on TiO2 nanoparticles

Hybrid photocatalysts consisting of molecular catalysts and solid-state surfaces have demonstrated great potential as robust and efficient systems in solar fuel production. Based on our prior work, we synthesized hybrid photocatalysts by depositing a macrocyclic Co(III) complex on three different TiO2 nanomaterials via a microwave method. The hybrid photocatalysts were tested in CO2 reduction and were thoroughly characterized with spectroscopic (UV–vis, FTIR and EPR) and microscopic (TEM) techniques. The presence of terminal OH groups on TiO2 surfaces was essential for the molecular deposition of catalytically active Co(III) sites. On a TiO2 material without such terminal OH groups, the Co(III) complex formed amorphous aggregates, which hindered interfacial electron transfer from photoactivated TiO2 to the surface molecular complex. EPR studies further revealed important information regarding the coordination geometry and interaction with CO2 of surface cobalt sites in the hybrid photocatalysts.

Effects of impregnation strategy on structure and performance of bimetallic CoFe/AC catalysts for higher alcohols synthesis from syngas

Higher alcohols synthesis from syngas via Fischer-Tropsch reaction has been studied on activated carbon supported bimetallic CoFe (denoted as CoFe/AC) catalysts. The effect of impregnation strategy on the structure and performance of the catalysts was investigated by means of N2 adsorption, HRTEM, ex-situ/in-situ XRD, CO adsorption, CO-TPD, XPS and CO hydrogenation. The results showed that the sequentially impregnated 15Co-5Fe/AC catalyst prepared with iron precursor firstly and then cobalt precursor had the highest CO uptake, metal dispersion and the maximum cobalt surface content, resulting in the enhancement of CO hydrogenation activity as well as the yield of mixed alcohols. On the other hand, the selectivity towards higher alcohols (C2+) was hardly affected by the impregnation sequence. It was found that the activated carbon facilitated the formation of Co2C species and doping of Fe into the 15Co/AC catalyst promoted the formation of Co-Fe alloy species. Both of Co2C and Co-Fe alloy species were observed in all the co-impregnated and sequentially impregnated catalysts after reaction. It was inferred that Co/Co2C as well as Co-Fe alloy species might be responsible for the synthesis of alcohols via CO hydrogenation.

High speed CO2 laser surface modification of iron/cobalt co-doped boroaluminosilicate glass and the impact on surface roughness, gloss and wettability

A preliminary study into the impact of high speed laser processing on the surface of iron and cobalt co-doped glass substrates using a 60 W continuous wave (cw) CO2 laser. Two types of processing, termed fill-processing and line-processing, were trialled. In fill-processed samples the surface roughness of the glass was found to increase linearly with laser power from an Sa value of 20.8 nm–2.1 μm at a processing power of 54 W. With line processing, a more exponential-like increase was observed with a roughness of 4 μm at 54 W. The change in surface properties of the glass, such as gloss and wettability, have also been measured. The contact angle of water was found to increase after laser processing by up to 64°. The surface gloss was varied between 45 and 100 gloss units (GUs).

Adsorption and decomposition of H2O on cobalt surfaces: A DFT study

Water adsorption and dissociation on clean and O-covered Co(100), Co(110) and Co(111) surfaces are studied using the density functional theory calculations. The results indicate that molecular water weakly binds to the surfaces and is feasible to desorption from the clean surfaces. Moreover, the pre-adsorption of O atom increases the binding of water to the surfaces, and prominently decreases the activation barriers of water dissociation into OH, especially on Co(110) surface. In contrast, the activation barrier for OH dissociation is slightly affected in the presence of O atom. Overall, this study reveals that O-assisted H2O favorably adsorbs dissociatively, forming OH chemisorbed on the surfaces, which further hinders H2O dissociation, and also illustrates the fact that molecular water dissociation is structure-sensitive on metal surfaces.

Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics

Operando DRIFT analysis of the surface species observed on Fischer Tropsch catalysts at work has been performed under H2/CO syngas at 503K. Decomposition of the evolution in the IR operando spectra has been done by chemometric analysis in order to depict the modifications occurring on the surface of the nanoparticles as a function of time on stream. Results obtained on three catalysts with different reducibility determined from temperature programmed reduction experiments are confronted in order to correlate the spectral observations to the cobalt surface atoms speciation. Attribution of the IR components in the 2100–1750cm−1 range is confirmed on the basis of the chemometric decomposition. The component falling in the 2068–2064cm−1 range is assigned to CO sorbed onto Co° surface atoms of partially H2-reduced nanoparticles. This component is found to progressively decrease as a function of catalyst time on stream which indicates a progressive reduction of the nanoparticles under syngas at 503K.