CO Monolayer Research Articles

CO Inversion on a NaCl(100) Surface: A Multireference Quantum Embedding Study.

We develop a multireference quantum embedding model to investigate a recent experimental observation of the isomerization of vibrationally excited CO molecules on a NaCl(100) surface [Science 2020, 367, 175-178]. To explore this mechanism, we built a reduced potential energy surface of CO interacting with NaCl(100) using a second-order multireference perturbation theory, modeling the adsorbate-surface interaction with our previously developed active space embedding theory (ASET). We considered an isolated CO molecule on NaCl(100) and a high-coverage CO monolayer (1/1), and for both we generated potential energy surfaces parametrized by the CO stretching, adsorption, and inversion coordinates. These surfaces are used to determine stationary points and adsorption energies and to perform a vibrational analysis of the states relevant to the inversion mechanism. We found that for near-equilibrium bond lengths, CO adsorbed in the C-down configuration is lower in energy than in the O-down configuration. Stretching of the C-O bond reverses the energetic order of these configurations, supporting the accepted isomerization mechanism. The vibrational constants obtained from these potential energy surfaces show a small (< 10 cm-1) blue- and red-shift for the C-down and O-down configurations, respectively, in agreement with experimental assignments and previous theoretical studies. Our vibrational analysis of the monolayer case suggests that the O-down configuration is energetically more stable than the C-down one beyond the 16th vibrational excited state of CO, a value slightly smaller than the one from quasi-classical trajectory simulations (22nd) and consistent with the experiment. Our analysis suggests that CO-CO interactions in the monolayer play an important role in stabilizing highly vibrationally excited states in the O-down configuration and reducing the barrier between the C-down and O-down geometries, therefore playing a crucial role in the inversion mechanism.

Influence of interfacial water and cations on the oxidation of CO at the platinum/ionic liquid interface.

CO oxidation is fundamental to the development of new catalyst materials for fuel cells and key for complete oxidation of small alcohols like methanol or ethanol on Pt catalysts. So far, room-temperature ionic liquids (RTIL) have been used to modify the selectivity and activity in electrocatalysis. In order to understand the mechanism of CO oxidation in RTIL in more detail we have investigated this reaction at the Pt(111)/1-butyl-3-methylimidazolium trifluorosulfonylimide [BMIM][NTf2] electrode/electrolyte interface as a function of H2O concentration and electrode potential with in situ sum-frequency generation (SFG) spectroscopy and infrared absorption spectroscopy (IRAS). Using SFG spectroscopy, we address the changes of linearly bonded CO molecules on Pt(111), while we monitor the changes in the bulk electrolyte with IRAS through vibrational bands from H2O, CO2 and CO. The presence of water in [BMIM][NTf2] shifts the onset potential for CO oxidation by more than 200 mV when the water concentration is increased from 0.01 to 1.5 M, which we relate to the incorporation and the availability of water at the electrode/electrolyte interface. The nature of the RTIL cation has also a large effect on the surface excess of H2O since RTILs like [BMMIM][NTf2] and [BMPyrr][NTf2] which are prone to form closed-packed structures, can block the incorporation of water and lead to more sluggish CO oxidation with larger overpotentials and oxidation in a much wider potential range for which we provide evidence by additional SFG measurements. These results clearly show that the choice of the RTIL is important for CO oxidation on Pt(111) electrode surfaces - an observation that is likely highly relevant also to other catalysts and catalytic reactions that require the presence of interfacial water.

Carbon-oxygen surface formation enhances secondary electron yield in Cu, Ag and Au

First-principles calculations coupled with Monte Carlo simulations are used to probe the role of a surface CO monolayer formation on secondary electron emission (SEE) from Cu, Ag, and Au (110) materials. It is shown that formation of such a layer increases the secondary electron emission in all systems. Analysis of calculated total density of states (TDOS) in Cu, Ag, and Au, and partial density of states (PDOS) of C and O confirm the formation of a covalent type bonding between C and O atoms. It is shown that such a bond modifies the TDOS and extended it to lower energies, which is then responsible for an increase in the probability density of secondary electron generation. Furthermore, a reduction in inelastic mean free path is predicted for all systems. Our predicted results for the secondary electron yield (SEY) compare very favorably with experimental data in all three materials, and exhibit increases in SEY. This is seen to occur despite increases in the work function for Cu, Ag, and Au. The present analysis can be extended to other absorbates and gas atoms at the surface, and such analyses will be present elsewhere.

Gas Sensing Mechanism and Adsorption Properties of C2H4 and CO Molecules on the Ag3-HfSe2 Monolayer: A First-Principle Study.

The detection of dissolved gases in oil is an important method for the analysis of transformer fault diagnosis. In this article, the potential-doped structure of the Ag3 cluster on the HfSe2 monolayer and adsorption behavior of CO and C2H4 upon Ag3–HfSe2 were studied theoretically. Herein, the binding energy, adsorption energy, band structure, density of state (DOS), partial density of state (PDOS), Mulliken charge analysis, and frontier molecular orbital were investigated. The results showed that the adsorption effect on C2H4 is stronger than that on CO. The electrical sensitivity and anti-interference were studied based on the bandgap and adsorption energy of gases. In particular, there is an increase of 55.49% in the electrical sensitivity of C2H4 after the adsorption. Compared to the adsorption energy of different gases, it was found that only the adsorption of the C2H4 system is chemisorption, while that of the others is physisorption. It illustrates the great anti-interference in the detection of C2H4. Therefore, the study explored the potential of HfSe2-modified materials for sensing and detecting CO and C2H4 to estimate the working state of power transformers.

Spin reorientation transition in ultrathin Co films on the vicinal surface Au(788)

In this work, we investigate the structural, morphological and magnetic properties of epitaxial Cobalt ultrathin films grown on the vicinal surface Au(788). The aim was to study the magnetization reversal and determine the influence of the regular arrangement of atomic steps, typical of a vicinal surface. The influence of the Co thickness on the spin reorientation from out-of-plane to in-plane magnetization were investigated by means of the magneto-optical Kerr effect (MOKE). Preparation and surface quality of the Co films were characterized using the surface experimental techniques LEED and STM. A smooth spin reorientation transition, between 8 and 12 atomic monolayers (ML) of Co, was observed from out-of-plane magnetization, between 5 ML and 12 ML of Co, to in-plane magnetization, above 13 ML of Co. Furthermore, the angular dependence of the magnetic hysteresis loops indicates uniaxial magnetic anisotropy parallel to the atomic steps in the surface plane of the system Au/Co/Au(788) for thicknesses between 13 ML and 20 ML of Co.

All-optical spin switching probability in [Tb/Co] multilayers

Since the first experimental observation of all-optical switching phenomena, intensive research has been focused on finding suitable magnetic systems that can be integrated as storage elements within spintronic devices and whose magnetization can be controlled through ultra-short single laser pulses. We report here atomistic spin simulations of all-optical switching in multilayered structures alternating n monolayers of Tb and m monolayers of Co. By using a two temperature model, we numerically calculate the thermal variation of the magnetization of each sublattice as well as the magnetization dynamics of [text {Tb}_n/text {Co}_m] multilayers upon incidence of a single laser pulse. In particular, the condition to observe thermally-induced magnetization switching is investigated upon varying systematically both the composition of the sample (n,m) and the laser fluence. The samples with one monolayer of Tb as [text {Tb}_1/text {Co}_2] and [text {Tb}_1/text {Co}_3] are showing thermally induced magnetization switching above a fluence threshold. The reversal mechanism is mediated by the residual magnetization of the Tb lattice while the Co is fully demagnetized in agreement with the models developed for ferrimagnetic alloys. The switching is however not fully deterministic but the error rate can be tuned by the damping parameter. Increasing the number of monolayers the switching becomes completely stochastic. The intermixing at the Tb/Co interfaces appears to be a promising way to reduce the stochasticity. These results predict for the first time the possibility of TIMS in [Tb/Co] multilayers and suggest the occurrence of sub-picosecond magnetization reversal using single laser pulses.

On the structure sensitivity of and CO coverage effects on formic acid decomposition on Pd surfaces

Using density functional theory calculations, the Pd-catalyzed vapor-phase formic acid decomposition was studied, with a focus on the structure sensitivity and CO coverage effects. A comprehensive reaction network was developed on both the (111) and (100) facets of Pd, at CO coverages of 0 and 5/9 monolayer (ML). Pd(100) was determined to be more reactive than Pd(111) at both CO coverages. The introduction of 5/9 ML CO decreased the activity of both facets significantly, due to destabilization of the surface intermediates and transition states on the CO-decorated surfaces. Three reaction pathways were explored on the clean surfaces: the formate (HCOO) pathway, the carboxyl (COOH) pathway leading to the formation of CO2, and the COOH pathway leading to the formation of CO (COOH→CO). Based on the DFT-derived energetics alone, it appears that all three pathways contribute to the reaction on clean Pd, whereas the presence of 5/9 ML of CO inhibits the HCOO pathway on both facets and favors the COOH→CO pathway on the (111) facet, but the COOH→CO2 one on the (100) facet. Moreover, at high CO coverages, alternative spectator CO-assisted adsorbate decomposition pathways were discovered, which could potentially play a role in formic acid decomposition on Pd catalysts under realistic reaction conditions.

Critical magnetic behavior in [Ag8/Co0.5]x64, [Ag8/Co1]x32 and [Ag16/Co1]x32 epitaxial multilayers

We investigate the low temperature magnetic behavior of three epitaxial Co/Ag multilayers, grown onto MgO (001) substrates, with a nominal content per period of either half a monolayer or one monolayer of Co, and either 8 or 16 Ag monolayers. The samples were studied by X-ray reflectivity and diffraction, transmission electron microscopy, magnetometry and ac susceptometry. The results indicated a well defined stacking sequence in the growth direction, the number of periods and of Ag monolayers per period being coincident with the nominal values for each sample. The Co layers were found to be discontinuous and corresponded to a quasi-monodisperse in-plane distribution of Co nanoparticles embedded in a Ag(001) matrix. The zero-field cooled and field cooled temperature variations of the low field magnetization indicated the presence of irreversibilities at temperatures below 20 K. The ac field frequency (f) and temperature (T) dependencies of the real part of the susceptibility (χ′) corresponded to a Vogel–Fulcher behavior in the three samples, and indicated a frequency shift parameter (Γ) of the order of 4 x 10-2. For each sample, the experimental data corresponding to the variations of the imaginary part of the ac susceptibility (χ″) with f and T were found to collapse into a single curve according to the dynamic scaling law. Taken together, these results allow us to conclude that the three multilayers experience a phase transition of the paramagnetic to superspin glass type, driven by the dipolar interactions between the Co nanoparticles. Regarding the influence of the multilayer features, we found a clear dependence of the order parameter of the transition on the nominal number of Co monolayers per period.

CO adsorption on MnO(100): Experimental benchmarks compared to DFT

CO adsorption on the MnO(100) surface was studied using temperature programmed desorption (TPD) and density functional theory (DFT). TPD results show that CO is weakly-bound on MnO(100), with an experimental adsorption energy of -35.6 ± 2.1 kJ/mol at terrace sites in the low coverage limit. PBE simulations suggest that CO adsorption causes an implausible (2 × 2) surface reconstruction. PBE+U simulations show no signs of surface reconstruction, and provide an accurate estimate of the adsorption energy (-36.4 kJ/mol) when combined with the DFT-D3 method with Becke-Jonson damping to correct for van der Waals interactions. This simulation also shows that CO adsorbs C-down onto the Mn2+ terrace site in a tilted geometry, which is also observed experimentally and computationally on the similarly-structured NiO(100) transition metal oxide surface. TPD results for large doses show a plateauing of the coverage at about 0.4 monolayers of CO at 85 K, with a defect coverage equivalent to 0.08 monolayers. Adsorption associated with defect sites is indicated by a high-temperature desorption tail which is not satisfactorily explained by DFT simulations of simple step or oxygen vacancy defects.

Efficient Electrochemical/Electroless Deposition Synthesis Routines for CO Tolerant Catalysts

Composite catalysts containing Pt and Mo-oxides are promising materials for anodic catalyst preparation in PEMFCs due to their high tolerance for CO-poisoning. The search for optimal preparation methods for these materials is an important task in modern electrochemical energetics.Two synthetic routines, namely electrodeposition and deposition under open-circuit conditions, were used for Pt-MoO x catalyst preparation. The prepared catalysts were characterized by a variety of modern physical methods (XRD, XPS, SEM, EDX); it was established that the material prepared by electroless deposition contains platinum shell upon platinum core. The materials obtained were tested in the reactions of CO adsorbate electrodeposition and dissolved CO-oxidation.It was found that molybdenum oxides in the composites prepared by electrodeposition broaden the major peak of CO-oxidation, but the peak maximum does not shift toward less positive values. Therefore, the CO monolayer is destabilized as a result of the contact with molybdenum oxides; however, only a small part of CO can be oxidized at these low potentials. Nevertheless, these catalysts are suitable for dissolved carbon monoxide oxidation. aPt·b(H x -2MoO3) composites were synthesized by the redox reaction of K2PtCl4 with reduced forms of hydrogen containing molybdenum bronzes. The CO desorption peak is broadened and shifted toward less positive potentials. aPt·b(H x -2MoO3) catalysts demonstrate high catalytic activity in the reaction of both adsorbed and dissolved CO oxidation. Therefore, one can conclude that both electrodeposition and electroless deposition are suitable methods for preparing PEMFC anode catalysts. Figure 1

Enabling Real-Time Detection of Electrochemical Desorptionphenomena with Sub-Monolayer Sensitivity

Across many scientific and industrial fields, it is a challenge to analyze chemical reaction products with high sensitivity and fast time-response. This is especially true in systems related to electrochemical energy conversion and storage, such as batteries and fuel cells, where accurate monitoring of fast and transient reaction phenomena often reveals key insights.Here, we present a unique analysis instrument using a microchip-based inlet system to couple a mass spectrometer directly to any liquid test environment, aqueous or non-aqueous, and exhibits a sensitivity 3-4 orders of magnitude than conventional differential electrochemical mass spectrometry (DEMS).1–3 The instrument allows to measure the evolution of volatile products and consumption of reactants at electrode surfaces. Due to its extraordinary sensitivity, the system can measure all the individual volatile molecules desorbing from an electrode surface during a single electrochemical turnover. Product formation can be measured from total Faradaic currents of 1 mA all the way down to 1 nA, corresponding to approximately 10 ppm of a monolayer desorbing from the electrode surface in 1s. These features enable time-resolved, fully quantitative measurements of transient phenomena during electrochemistry, providing fundamental insight in the electrochemical reaction mechanisms. Furthermore, an accurately calibrated on-chip gas system allows rapid gas switching between different gases, both inert and reactive. Due to the near-instantaneous equilibration between gas and electrolyte, the transient response of electrodes to rapid gas exposure changes can be measured.The capabilities of this analysis system are showcased by various examples, including electrochemical water-splitting at low overpotential and low-surface area CO-stripping. In situ quantification of hydrogen and oxygen evolution is shown at Faradaic currents on the order of 1 nA. In comparison, conventional techniques can only detect down to about 10 μA of continuous product formation. Finally, the electrochemical stripping of < 1 % of a CO monolayer at standard potential scan-rates of 50 mV/s is shown (Fig. 1).Figure 1. H2 evolution and CO stripping experiments using a polycrystalline Pt electrode in 1M HClO4. Trimarco, D. B. et al. Enabling real-time detection of electrochemical desorption phenomena with sub-monolayer sensitivity. Electrochim. Acta 268, 520–530 (2018).Roy, C. et al. Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeOxHy. Nat. Catal. 1, 820–829 (2018).Winiwarter, A. et al. Towards an atomistic understanding of electrocatalytic partial hydrocarbon oxidation: propene on palladium. Energy Environ. Sci. (2019) doi:10.1039/C8EE03426E. Figure 1

Spin-orbit-proximitized ferromagnetic metal by monolayer transition metal dichalcogenide: Atlas of spectral functions, spin textures, and spin-orbit torques in Co/MoSe2, Co/WSe2 , and Co/TaSe2 heterostructures

The bilayer heterostructures composed of an ultrathin ferromagnetic metal (FM) and a material hosting strong spin-orbit (SO) coupling are principal resource for SO torque and spin-to-charge conversion nonequilibrium effects in spintronics. We demonstrate how hybridization of wavefunctions of Co layer and a monolayer of transition metal dichalcogenides (TMDs)---such as semiconducting MoSe$_2$ and WSe$_2$ or metallic TaSe$_2$---can lead to dramatic transmutation of electronic and spin structure of Co within some distance away from its interface with TMD, when compared to the bulk of Co or its surface in contact with vacuum. This is due to proximity induced SO splitting of Co bands encoded in the spectral functions and spin textures on its monolayers, which we obtain using noncollinear density functional theory (ncDFT) combined with equilibrium Green function (GF) calculations. In fact, SO splitting is present due to structural inversion asymmetry of the bilayer even if SO coupling within TMD monolayer is artificially switched off in ncDFT calculations, but switching it on makes the effects associated with proximity SO coupling within Co layer about five times larger. Injecting spin-unpolarized charge current through SO-proximitized monolayers of Co generates nonequilibrium spin density over them, so that its cross product with the magnetization of Co determines SO torque. The SO torque computed via first-principles quantum transport methodology, which combines ncDFT with nonequilibrium GF calculations, can be used as the screening parameter to identify optimal combination of materials and their interfaces for applications in spintronics. In particular, we identify heterostructure two-monolayer-Co/monolayer-WSe$_2$ as the most optimal.

The coverage dependence of the infrared absorption of CO adsorbed to NaCl(100).

CO adsorbed to NaCl(100) exhibits perhaps the weakest possible coupling between the adsorbate and solid. It is, therefore, an ideal system to observe the influence of adsorbate-adsorbate interactions on infrared absorption. In this work, we report polarized FTIR absorption spectra of CO/NaCl(100) as a function of coverage (0.02 ≤ θ ≤ 1 ML), where the coverage has been quantitatively determined by temperature-programmed desorption and molecular beam dosing. We extend a previous semi-empirical model designed to describe the screening of the local electric field due to dipole-dipole interactions in a CO monolayer. The extended model applies to sub-monolayer coverages and describes properly the electric field of the absorbed radiation at the vacuum-substrate interface. Fitting this model to coverage-dependent IR absorption data allows us to derive the vibrational and electronic polarizabilities [χv = 0.0435(14) Å3, χe = 3.30(36) Å3] and the integrated absorption cross section of 2.51(8) × 10-17 cm/molecule for an isolated CO molecule adsorbed at the NaCl (100) surface. The determined integrated absorption cross section is substantially smaller than that of gas phase CO.

Surface-structure sensitive chemical diffusivity and reactivity of CO adsorbates on noble metal electrocatalysts

The electrooxidation of carbon monoxide is a typical example of structure sensitivity in electrocatalysis for fuel cells. Understanding the structure sensitivity is crucial in designing highly efficient catalysts and elucidating reaction mechanisms. Conventionally, only the reactivity of active sites was considered and optimized. However, how the reactant chemically moves to the active site has not been well studied. In this work, we found that COad hopping is a general phenomenon on noble metal surfaces such as platinum, iridium, rhodium, and palladium in the case of CO monolayer electrooxidation. Differences in the performances of COad stripping on platinum nanoparticles with different surface structures reveal that the COad hopping rate is structure sensitive as well. Assisted by a diffusion modeling and density functional theory calculations, it is demonstrated that the structure sensitivity is as a result of a compromise between COad chemical diffusivity and reactivity with OHad.

Adsorption and diffusion behaviors of H2, H2S, NH3, CO and H2O gases molecules on MoO3 monolayer: A DFT study

Molybdenum trioxide (MoO3) with α-phase is a promising material for gas sensing because of its high sensitivity, fast response and thermodynamic stability. To probe the mechanism of superior gas detection ability of MoO3 monolayer, the adsorption and diffusion of H2, H2S, NH3, CO and H2O molecules on two-dimensional (2D) MoO3 layer are studied via density functional theory (DFT) calculations. Based on calculated adsorption energies, density of states, charge transfer, diffusion barriers and diffusion coefficient, MoO3 shows a superior sensitive and fast response to H2 and H2S than CO, NH3, H2O, which is consistent with experimental conclusions. Moreover, the response of MoO3 to H2S and H2 will be obviously enhanced at high gas concentration, and the incorporation of H2 and H2S results in an obvious increasing in DOS near Fermi level. Our analysis provides a conceptual foundation for future design of MoO3-based gas sensing materials.

Revisiting Electrodeposited Inverse Opals: New Insights into Magnetic Frustration

Electrochemical deposition into templated architectures has been shown to increase the observed saturation magnetization of a range of magnetic materials (e.g. 1). Here we have created monolayer (2D) and 3D opal-type structures of NiFe (permalloy) and Co by electrodeposition into self-assembled polymer templates. The microstructure and thickness of the resultant macroporous structures is controlled by the electrochemical parameters during nucleation and diffusion-controlled growth. Preferential crystallographic texture is observed – resulting in magnetic anisotropy for Co. The magnetic behaviour is assessed by both conventional bulk magnetometry, and locally using high-resolution Lorentz microscopy in the transmission electron microscope (TEM) - providing new insights into the functional properties of these materials. Hao, F. Q. Zhu, C. L. Chien & P. C. Searson. Fabrication and magnetic properties of ordered macroporous nickel structures. J. Electrochem. Soc. 154, D65-D69, (2007)

Adsorption of small molecules on a [formula omitted] CO monolayer

The uptake of pollutant gases from the environment is an important topic in nanomaterial science. Although this issue has been extensively studied from all aspects, there is still a need for efficient adsorbent substrates. As carbon-based materials are reliable for removal of gas molecules, we studied the capacity of the recently predicted monolayer of Pmma CO for detection of gas molecules, especially for uptake of H2S. Monolayer Pmma CO was found to be a stable material with unexpected physical and chemical properties. Motivated by this, on the basis of the first-principles plane-wave method in density functional theory, using the generalized gradient approximation, we chose pristine monolayer Pmma CO and two different defective Pmma CO structures as substrates and investigated the adsorption behavior of the H2S molecule on these monolayers. Moreover, we calculated the binding energies of other gas molecules, such as CO, CO2, NO, and NO 2, on pristine monolayer Pmma CO. We also examined the properties of the point-defect structures of monolayer Pmma CO in detail. As the calculated binding energies of H2S on the chosen substrates are moderate, we investigated the adsorption of H2S on Ni-doped Pmma CO. We found that the binding energy for the toxic gas H2S greatly increases on the Ni-doped Pmma CO structure. Our results show that monolayer Pmma CO can be modified by point defects, specifically by atom doping. Thus, the Ni-doped Pmma CO substrate could be an alternative medium for detection of H2S.

Effect of modification of multi-walled carbon nanotubes with nitrogen-containing polymers on the electrochemical performance of Pt/CNT catalysts in PEMFC

The influence of the surface modification of multi-walled carbon nanotubes (MWCNT) with nitrogen-containing compounds on the performance of 40 wt% Pt/MWCNT catalysts in the oxygen electroreduction reaction (ORR) was investigated using a rotating disk electrode (RDE) at 10–35 °C in 0.1 M HClO4 as electrolyte in electrochemical cell, and in a hydrogen–oxygen polymer electrolyte membrane fuel cell (PEMFC) at 82 °C. The catalysts were characterized by low-temperature nitrogen adsorption, XPS, TEM, gas-phase CO titration, electrooxidation of the adsorbed CO monolayer, and cyclic voltammetry. It was shown that the modification of MWCNT with melamine–formaldehyde resin leads to the surface nitrogen concentration up to 8.3 at.% (CNT-MF sample). The 40 wt% Pt/CNT-MF catalyst with 0.1 mg cm−2 Pt loading on the cathode showed a good performance in PEMFC (~ 0.61 W cm−2) and a high utilization ratio (0.84) of Pt in membrane electrode assembly as compared to Pt/CNT catalyst (~ 0.37 W cm−2 and utilization of 0.29). The higher power density of nitrogen-modified catalysts was ascribed to a higher utilization of Pt in the electrode layer.

Elucidating the structure and dynamics of CO ad-layers on MgO surfaces.

The combination of quantum-mechanical simulations and infrared absorption spectroscopy measurements provides a clear picture for a long standing puzzle in surface science: the actual structure and vibrational dynamics of the low-temperature ordered CO monolayer adsorbed on (001) MgO surfaces. The equilibrium structure of the commensurate (4 × 2) adsorbed phase consists of three CO molecules per primitive cell (surface coverage of 75%) located at two inequivalent sites: one molecule seats upright on top of a Mg site while two molecules, tilted off the normal to the surface, are symmetrically positioned relative to the upright one with anti-parallel projections on the surface. This configuration, long believed to be incompatible with measured polarization infrared spectra, is shown to reproduce all observed spectral features, including a new, unexpected one: the vanishing anharmonicity of CO stretching modes in the monolayer.

Atomically Defined Iron Carbide Surface for Fischer–Tropsch Synthesis Catalysis

With the purpose of investigating the reactivity of Fe carbide as an active phase in Fischer–Tropsch catalysis, we studied the formation of a well-defined Fe carbide surface structure resulting from carbon exposure of an Fe film on Au(111). Using two different sources of carbon (C), namely atomic carbon and ethylene gas, we used synchrotron X-ray photoelectron spectroscopy (XPS) to show that a 6 ML Fe film readily converts into a well-defined and thermodynamically stable carbide phase. Scanning tunneling microscopy (STM) showed that the surface of the Fe carbide film is crystalline and is dominated by Fe(110)-like facets perturbed into a (2 × 2) periodic structure due to insertion of C in the interstitial sites. The reactivity of the carbide film toward CO, H2, and O2 was furthermore probed by XPS under vacuum conditions. While the pristine Fe carbide surface was unreactive toward hydrogen gas at 500 K, we interestingly found that CO dissociation from a preadsorbed monolayer of CO takes place already at low temperature. This observation points to an intrinsic activity of the Fe carbide phase where additional carbon originating from CO can be placed in the Fe carbide surface. The catalytic significance of the model catalyst surface presented here is that it can be seen as a stable Fe carbide phase with intrinsically vacant sites for additional C insertion at elevated pressure, and we propose that such additional C may act as active species in C–C coupling reactions during FTS. The studies pave the way for a better understanding of FTS processes on Fe-based catalysts on the basis of a well-defined model surface.