High CO Coverage Research Articles

Nickel Dynamics Switches the Selectivity of CO2 Hydrogenation

AbstractThe Reverse Water Gas‐Shift reaction (CO2+H2 CO+H2O) allows to balance syn‐gas under industrial conditions. Nickel has been suggested as a potential catalyst but the temperature required is too high, more than 800 °C, limiting practical implementation but when lowering the temperature methanation occurs. Simulations via Density Functional Theory on well‐defined surfaces have systematically failed to reproduce these experimental results. But under reaction conditions, Ni surfaces are not static and DFT models coupled to microkinetics show that low temperatures (high CO coverages) drive the generation of Ni adatoms that are the active sites for methanation. At higher temperatures, the adatom population decreases, and the selectivity towards CO increases. Thus the mechanism behind the selectivity switch is driven by the dynamics induced by reaction intermediates. Our work contributes to the inclusion of dynamic aspects of materials under reaction conditions in the understanding of complex catalytic behaviour.

Nickel Dynamics Switches the Selectivity of CO2 Hydrogenation.

The Reverse Water Gas-Shift reaction (CO2 + H2=CO + H2O) allows to balance syn-gas under industrial conditions. Nickel has been suggested as a potential catalyst but the temperature required is too high, more than 800ºC, limiting practical implementation but when lowering the temperature methanation occurs. Simulations via Density Functional Theory on well-defined surfaces have systematically failed to reproduce these experimental results. But under reaction conditions Ni surfaces are not static and DFT models coupled to microkinetics show that at high CO coverages drive the generation of Ni adatoms that are the active sites for methanation. At higher temperatures, the adatom population decreases, and the selectivity towards CO increases. Thus, the mechanism behind the selectivity switch is driven by the dynamics induced by reaction intermediates. Our work contributes to the inclusion of dynamic aspects of materials under reaction conditions in the understanding of complex catalytic behaviour.

Copper nanoclusters derived from copper phthalocyanine as real active sites for CO2 electroreduction: Exploring size dependency on selectivity ‐ A mini review

AbstractThe electrochemical reduction reaction of CO2 (CO2RR) holds promise for converting CO2 into valuable fuels and chemicals, particularly when powered by renewable electricity, thereby aiding in reducing atmospheric CO2 levels and addressing climate change. Copper phthalocyanine and its derivatives (Cu‐Pcs) have attracted significant attention as versatile electrocatalytic materials with high selectivity toward various hydrocarbon products. However, the real active sites of Cu‐Pcs for different products vary, and there is a lack of comprehensive summary. To address this gap, we analyze and summarize previous research, yielding the following insights: Cu‐Pcs undergo reconstruction and demetallization during CO2RR, with Cu(II) converting to Cu(0), forming transient copper nanoclusters (Cu NCs). The selectivity for CO2RR products closely correlates with the size of those derived Cu NCs. Specifically, reversible Cu NCs with ultrasmall sizes (≤2 nm), which revert to Cu‐Pcs after electrolysis, exhibit high selectivity toward CH4. As Cu NCs increase in size, there is a higher CO coverage, promoting CO generation. When Cu NCs exceed a critical threshold size (approximately 15 nm), C‐C coupling can occur, facilitating the formation of multicarbon (C2+) products. Furthermore, the structure of macrocycles, types of functional groups, and properties of carbon substrates influence the size and electron density of Cu NCs, thereby impacting the selectivity of CO2RR products.

Coverage-dependent activation of CO over Ni/Cu(100) single atom alloys (SAAs).

Single atom alloys (SAAs) often bring new chemistry in heterogeneous catalysis and well-defined structure for the study of structure-activity relationship (SAR). However, the existing pressure gap causes the reported SARs quite divergent. Herein, we have studied CO activation over Ni/Cu(100) SAAs in ultrahigh vacuum (UHV) and millibar range. While the Ni SAAs formed on Cu(100) significantly enhance the CO adsorption strength under UHV conditions, the CO treatment at elevated pressure leads to notable surface carbon and oxygen deposition through surface reaction. Density functional theory calculations revealed that either dissociation or disproportionation is thermodynamically forbidden for the coverage of CO less than 5/16 ML. However, these two reaction pathways can be opened at higher CO coverages due to the elevated energy state involving repulsion between adsorbed CO. This work uncovers the initial activation process of CO and demonstrates one typical cause for the pressure gap in surface science study as well.

Solution of the structure of the high-coverage CO layer on the Ru(0001) surface—A combined study by density functional theory and scanning tunneling microscopy

Structures formed by dense CO adsorption layers can provide information about the balance between molecule–surface and molecule–molecule interactions. However, in many cases, the structure models are not clear. Using density functional theory (DFT) and scanning tunneling microscopy (STM), we have investigated the high-coverage CO layer on the Ru(0001) surface. Previous investigations by low-energy electron diffraction (LEED) and vibrational spectroscopy led to conflicting results about the structure. In the present study, 88 models with coverages between 0.58 and 0.77 monolayers have been analyzed by DFT. The most stable structures consist of small, compact CO clusters with an internal pseudo 1×1 structure. The CO molecules in the cluster centers occupy on-top sites in an upright position, whereas the molecules farther outside are slightly shifted from these sites and tilted outward. STM data of the CO-saturated surface at low temperatures, corresponding to a coverage of 0.66 monolayers, show a quasi-hexagonal pattern of features with an internal hexagonal fine structure. Simulated images based on the cluster model agree with the experimental data. It is concluded that the high-coverage CO layer consists of the close-packed clusters predicted by DFT as the most stable structure elements. In the experiment, the sizes and shapes of the clusters vary. However, the arrangement is not random but follows defined tiling rules. The structure remains ordered, almost up to room temperature. The LEED data are re-interpreted on the basis of the Fourier transforms of the STM data, solving the long-standing conflict about the structure.

Novel flexible aromatic Cu3 metal-organic π-cluster for electrocatalytic CO2 reduction reaction

Producing Cn products through electrocatalytic CO2 reduction reaction (CO2RR) is of great significance in addressing the global warming crisis. Organic-inorganic hybrid catalysts, characterized by precise and controllable active sites and metal-ligand synergistic interactions, can enhance the reaction activity and stability of Cu-based catalysts. Herein, based on density functional theory (DFT), a novel flexible aromatic Cu3 metal-organic π-cluster (Cu3-π cluster) was constructed, consisting of triple-atom active centers and pentalyne ligands. During the catalytic CO2RR process, the adsorption of H can promote the activation of CO2, this converts the competing hydrogen evolution reaction (HER) into promoting CO2RR. Enhanced aromaticity of its cluster core is credited with stabilizing the coadsorption of H and CO2 (H* + CO2*), consequently lowering the reaction free energy of the CO2 activation process. Research has shown that Cu3-π cluster have high catalytic activity for electrocatalytic CO2 generation of C2H4. Considering the solvation effect, the limit potential of this reaction is −0.60 V. Furthermore, the reaction free energies suggest that the Cu3-π cluster is more inclined to yield C2H4(g) products via COCO* coupling. Moreover, the high CO coverage at the triple-atom active centers not only makes it more challenging for this cluster to adsorb H, but also reduces the energy barrier of the COCO* coupling reaction.In the entire reaction pathway of C2H4(g), there exhibits dynamic self-adaptive behavior in the bond lengths and bond angles of the three Cu atoms in the cluster core, leading to fluctuations in aromaticity. The flexibility and aromaticity changes in this structure enable the Cu3-π cluster to better stabilize intermediates. This work provides theoretical guidance for the application of metal-organic π-clusters, accelerates the screening of catalysts for CO2RR, and provides powerful theoretical guidance for the structure-activity relationships between aromaticity and catalytic activity.

Sequential deposition of FeNC–Cu tandem CO2 reduction electrocatalysts towards the low overpotential production of C2+ alcohols

Tandem CO2 reduction electrocatalysts that combine a material that selectively produces CO with Cu are capable of producing hydrocarbons at low overpotentials and high selectivity. However, controlling the spatial distribution and the catalytic activity of the CO-making catalyst remains a challenge. In this work, a novel tandem electrocatalyst that overcomes limitations of simple Cu catalysts, namely selectivity and efficiency at low overpotential, is presented. The tandem electrocatalysts are prepared through a sequential spray coating protocol, using a single atom Fe in N-doped C (FeNC) as the selective CO-producing catalyst and commercial Cu nanopowder. The high faradaic efficiency towards CO of FeNC (99% observed at −0.60 V vs. RHE) provides a high CO coverage to the Cu particles, leading to reduced hydrogen evolution and the selective formation of ethanol and n-propanol at a much low overpotential than that of bare Cu.

CO Dimerization on Cu Nanoparticles under High CO Coverage

CO Dimerization on Cu Nanoparticles under High CO Coverage

Boosting C–C coupling to multicarbon products via high-pressure CO electroreduction

Electrochemical CO reduction reaction (CORR) provides a promising approach for producing valuable multicarbon products (C2+), while the low solubility of CO in aqueous solution and high energy barrier of C–C coupling as well as the competing hydrogen evolution reaction (HER) largely limit the efficiency for C2+ production in CORR. Here we report an overturn on the Faradaic efficiency of CORR from being HER-dominant to C2+ formation-dominant over a wide potential window, accompanied by a significant activity enhancement over a Moss-like Cu catalyst via pressuring CO. With the CO pressure rising from 1 to 40 atm, the C2+ Faradaic efficiency and partial current density remarkably increase from 22.8% and 18.9 mA cm−2 to 89.7% and 116.7 mA cm−2, respectively. Experimental and theoretical investigations reveal that high pressure-induced high CO coverage on metallic Cu surface weakens the Cu–C bond via reducing electron transfer from Cu to adsorbed CO and restrains hydrogen adsorption, which significantly facilitates the C–C coupling while suppressing HER on the predominant Cu(111) surface, thereby boosting the CO electroreduction to C2+ activity.

Surface morphology of Fe3C catalyst under different CO coverage from DFT and thermodynamics

High-coverage CO adsorption on seven surfaces of Fe3C catalyst has been systematically studied by density functional theory and thermodynamics. Results show that CO molecules prefer to bind on surface iron atoms of Fe3C catalyst, and the saturation coverage ranges from 0.5 to 0.833 monolayers (ML) on different surfaces. The equilibrium phase diagrams of CO adsorption on different Fe3C surfaces at different CO coverage are plotted, and the stable CO concentration on each surface of Fe3C at any CO partial pressure and temperature could be easily identified. CO adsorption/activation on Fe3C catalyst via an acceptance-donation mechanism can be explained by the projected density of states (PDOS) and charge density difference plots. Interestingly, stabilities of the Fe3C surfaces can be modified by adsorbed CO, and eventually the morphologies of Fe3C nanoparticle can also be changed. Our results provide essential information for selective synthesis of Fe3C catalyst with specific structures experimentally.

Pressure dependence and mechanism of Mn promotion of silica-supported Co catalyst in the Fischer-Tropsch reaction

The mechanism of Mn promotion of a silica-supported Co catalyst in the Fischer-Tropsch reaction has been studied at varying pressures up to 20 bar. IR spectroscopy in combination with DFT calculations suggest adsorbed CO is activated by reaction with an oxygen vacancy in the MnO, which covers the Co surface. This leads to a higher activity, higher CHx coverage and thus higher C5+ and lower CH4 selectivity. Increasing the pressure magnifies the selectivity differences. However, above around 4 bar, the effect of Mn on the selectivities is reversed and the C5+ selectivity is decreased by Mn addition. This is tentatively attributed to Mn promoting the C-O bond dissociation but not the chain growth. Formed monomers have to migrate to stepped sites for chain growth on the Co surface. Whilst this is migration is not impeded by co-adsorbates at low pressure, migration could be hindered by especially the high CO coverage at high pressure.

Diffusion of O Atoms on a CO-Covered Ru(0001) Surface─A Combined High-Speed Scanning Tunneling Microscopy and Density Functional Theory Study at an Enhanced CO Coverage

The diffusion of adsorbed O atoms on a CO-covered Ru(0001) surface has recently been explained by a “door-opening” mechanism which is driven by fluctuations in the CO layer. Here, we analyze how this mechanism changes at a higher CO coverage than the 0.33 monolayers applied in the previous study and, therefore, lower concentrations of empty sites. High-speed scanning tunneling microscopy was used to track the O atoms. A statistical model based on nearest-neighbor jumps and uniform hopping probabilities into the six equivalent directions of the Ru(0001) lattice describes the data well. Surprisingly, the hopping rates, which involve site exchanges with CO molecules, are higher than the site exchanges at 0.33 monolayers. Density functional theory calculations were performed to explain these observations. The configurations of the CO molecules around the adsorbed O atoms obtained at the higher CO coverage are disordered. Displacements of CO molecules cost less energy than displacements in the ordered (3×3)R30° structure of the previous study. In addition, the activation energies for the jumps of the O atoms are lowered by enhanced O/CO repulsions. The atomic processes are thus faster at the higher coverage, but the general features of the door-opening mechanism remain valid.

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.

Si Doping-Induced Electronic Structure Regulation of Single-Atom Fe Sites for Boosted CO2 Electroreduction at Low Overpotentials.

Transition metal-based single-atom catalysts (TM-SACs) are promising alternatives to Au- and Ag-based electrocatalysts for CO production through CO2 reduction reaction. However, developing TM-SACs with high activity and selectivity at low overpotentials is challenging. Herein, a novel Fe-based SAC with Si doping (Fe-N-C-Si) was prepared, which shows a record-high electrocatalytic performance toward the CO2-to-CO conversion with exceptional current density (>350.0mAcm-2) and~100% Faradaic efficiency (FE) at the overpotential of <400mV, far superior to the reported Fe-based SACs. Further assembling Fe-N-C-Si as the cathode in a rechargeable Zn-CO2 battery delivers an outstanding performance with a maximal power density of 2.44mWcm-2 at an output voltage of 0.30V, as well as high cycling stability and FE (>90%) for CO production. Experimental combined with theoretical analysis unraveled that the nearby Si dopants in the form of Si-C/N bonds modulate the electronic structure of the atomic Fe sites in Fe-N-C-Si to markedly accelerate the key pathway involving *CO intermediate desorption, inhibiting the poisoning of the Fe sites under high CO coverage and thus boosting the CO2RR performance. This work provides an efficient strategy to tune the adsorption/desorption behaviors of intermediates on single-atom sites to improve their electrocatalytic performance.

Tuning product distributions of CO2 electroreduction over copper foil through cathodic corrosion

Electrocatalytic CO2 reduction is an effective way to close the global carbon cycle. Copper is a promising metal for CO2 conversion to multiple products but suffers from low selectivity. Here commercial copper foil is exploited as an efficient catalyst for CO2 reduction with tunable product distributions through electrochemical processing. Ultrafine Cu nanoparticles were generated with dominant (111) facets on the 2D Cu foil surface through cathodic corrosion, showing CH4 selectivity up to 69.6 %. Meanwhile, C2+ products became dominant with a selectivity up to 66 % on the same Cu (111) nanoparticles when supported on electrodeposited copper dendrites. The in-situ Raman spectroscopy indicated that the high CO coverage and local pH created by the hierarchical structure contributed to the product transformation from C1 to C2+. This study demonstrates that electrochemical processing could be employed as a promising method to control and tune the product selectivity of CO2 electroreduction.

Investigations of mechanism, surface species and support effects in CO hydrogenation over Rh

The Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates over Rh/ZrO2 and Rh/SiO2 was studied. Catalytic reaction tests show a support effect with a faster steady state reaction rate over Rh/ZrO2 compared to Rh/SiO2. Temperature programmed hydrogenation (TPH) experiments reveal that the CO dissociation on the metal surface is rate limiting, and the support effect thus accelerates the CO dissociation on the metal. Combined TPH and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies after low-temperature CO pre-adsorption reveal a H-assisted C-O bond breakage through CH3O species on the Rh surface. Transient measurements indicate that this mechanism is also likely to be in operation during the steady state reaction at higher temperatures, although here the methoxide is too short-lived to be detected at steady state. This hydrogen-assisted CO activation can help to explain that previous studies have observed an inverse H/D isotope effect for Rh despite CO dissociation being the rate limiting step. TPH studies show that both CO pre-adsorption at 30 ℃ and CO/H2 exposure at 250 ℃ lead to so high CO coverages that it restricts the CO activation, which only starts once part of the CO has desorbed. The near-complete CO coverage on the working Rh surface is restricting the rate, but is essential for the selectivity towards oxygenates. Studies of acetaldehyde conversion in various atmospheres over Rh/SiO2 and Rh/ZrO2 catalysts show that acetaldehyde decomposes over a bare Rh surface with a rate that greatly exceeds the oxygenate formation rate in CO hydrogenation. In the presence of CO the acetaldehyde decomposition is strongly inhibited. The high CO coverage on the surface of the working Rh catalysts thus prevents oxygenate decomposition and is therefore essential for the ability to produce oxygenated products. The reaction temperature is observed to play a role for the establishment of the high coverage. During exposure to CO/H2 at reaction temperatures (>200 ℃) an activated process occurs whereby both the stability and coverage of the CO adlayer increases. This activated stabilization of the CO adlayer shifts the CO activation up in temperature in a subsequent TPH. The results contribute to a fundamental understanding of the reaction mechanism and support effects in the Rh-catalyzed CO hydrogenation, which can assist the formulation of improved Rh-based catalysts. The results, such as the identification of a H-assisted mechanism of CO hydrogenenation via methoxide, could also be of general relevance for the understanding of CO hydrogenation over other metals.

Boudouard reaction under graphene cover on Ni(1 1 1)

We investigated the interaction of CO with graphene/Ni(111) and the Boudouard reaction at 3.7 mbar by Near Ambient Pressure X-Ray Photoemission Spectroscopy (NAPXPS), i.e. at one order of magnitude higher pressure than previously explored in-operando conditions. In this regime, CO intercalates under the graphene layer causing its partial detachment from the Ni substrate. The so-obtained high local CO coverage opens the way to CO2 formation via the Boudouard reaction. Its onset is witnessed by observing physisorbed CO2 accumulating below the graphene cover. The so-generated additional carbon atoms transform carbide into graphene, causing the expansion of the graphene islands. In addition, CO adsorption occurs on the strongly interacting areas of the graphene layer, confirming previous results obtained by some of us at low temperatures and in ultra-high vacuum conditions.

High current density microkinetic and electronic structure analysis of CO2 reduction using Co and Fe complexes on gas diffusion electrode

Reaction mechanisms of electrocatalytic CO 2 reduction into CO over Co or Fe complexes were examined using gas diffusion electrodes to meet the requirement of high current densities for industrial deployment. Our experimental and theoretical calculation results consistently revealed that the Fe-based molecular catalysts exhibited more positive redox potentials relevant to CO 2 electrocatalysis but disfavored the desorption of generated CO, especially at high overpotentials, failing to achieve appreciable reaction rates. Distinctively, the heterogenized Co-based molecular complexes were found to be tolerant to the high coverage of CO at steady state on the active site and achieved rates exceeding 100 mA cm −2 toward exclusive CO evolution. Density-functional theory calculations not only disclosed the redox non-innocent tetraphenylporphyrins and phthalocyanines during electrocatalytic CO 2 reduction but also corroborated the energetics, especially for CO 2 and CO adsorption, accounting for distinctive reaction pathways between Co and Fe complexes. • Microkinetic analysis was performed at high current densities • The redox non-innocent TPP and Pc were disclosed by electronic structure analysis • The distinctive performance between Co and Fe complexes was interpreted Molecular catalysts can electrocatalytically convert CO 2 reduction into CO at nearly 100% selectivity. However, the associated electrokinetic data and reaction mechanisms remain debatable, especially under higher current densities, hindering the establishment of design guidelines for effective catalysis. Herein, the reaction mechanism using Co and Fe complexes was examined using a gas diffusion electrode to meet the requirement of high current densities. The distinctive performance between Co and Fe complexes was detailed using microkinetic analysis and theoretical calculations. This work not only provides conclusive representation related to electronic structure and reaction mechanism of molecular complexes but also sheds light on the further development of immobilized molecular electrocatalysts at an industrial scale. The reactivity and energetics at each elementary step of immobilized Co and Fe complexes on gas diffusion electrode for the electrocatalytic CO 2 reduction reaction were examined through a combination of microkinetic assessment and theoretical DFT calculations. The experimental and theoretical calculation results disclosed distinctive performance between Co and Fe complexes resulting from their electronic structure and CO or CO 2 adsorption free energies on the metal center.

Chasing PtOx species in ceria supported platinum during CO oxidation extinction with correlative operando spectroscopic techniques

Industrially relevant, highly dispersed, Pt/ceria and reference Pt/alumina catalysts with narrow Pt particle size distributions have been prepared, characterised ex situ and studied for CO oxidation by operando infrared and X-ray absorption spectroscopy. At high CO conversions, spectator CO ad-species on ionic platinum are observed while the CO oxidation proceeds on Pt particles in a high oxidation state exhibiting significant PtO coordination. During the protracted catalytic extinction, the CO coverage builds up gradually while the Pt oxidation state and PtO coordination remain high because of interactions with ceria. The observed CO oxidation at high CO coverage is suggested to involve sites at the platinum-ceria boundary that cannot be CO self-poisoned. This behaviour is in stark contrast to that of Pt/alumina, which shows removal of platinum oxides formed during CO oxidation and the classical drop in catalytic activity caused by rapid CO self-poisoning when reaching a critical temperature.

CO adsorption on Co(0001) revisited: High-coverage CO superstructures on the close-packed surface of cobalt

The adsorbate overlayer structures formed by CO on highly covered Co(0001) were investigated using temperature programmed desorption, low energy electron diffraction, reflection absorption infrared spectroscopy and synchrotron-based x-ray photoemission spectroscopy, with the purpose to clarify some recent confusion on CO adsorption at high coverages. TPD shows that the coverage of the (2√3 × 2√3)R30o structure (abbreviated as 2√3 structure) is 7/12 (0.583) ML. Its unit cell contains one COtop and six CObridge species that form three ‘bridge dicarbonyls’ in which two CObridge adsorbates share the same cobalt surface atom. Between 0.58 and 0.63 ML the 2√3 structure breaks up into small 2D domains separated by antiphase domain boundaries that consist of ‘double bridge dicarbonyls’ in which three CObridge adsorbates share two cobalt surface atoms. A phase transition that occurs around 0.63 ML creates previously unknown c(8 × 2) and c(12 × 2) domain boundary structures which consist of high density strips with a (2 × 2)–3CO structure and top/hollow site occupation which are separated by antiphase domain boundaries with a lower local coverage. Up to 0.63 ML the structures found at low temperature in UHV are very similar to those observed by others using in-situ STM at 300 K under CO pressure. This changes above 0.63 ML, where STM shows that moiré structures form instead of the c(n × 2) domain boundary structures. We propose that the moiré and c(n × 2) structures have a very similar enthalpy of formation so that small variations of temperature and CO chemical potential can lead to different structures for the same CO coverage. We show that these high-density phases that exist above 0.5 ML do not form at the typical pressures and temperatures used in applied FTS, but they do need to be considered when CO adsorption at room temperature is used as a diagnostic tool to characterize the catalyst surface.