CO Surface Coverage Research Articles

Adsorbed Carbon Monoxide-Enabled Self-Terminated Au-Grafting on Pt6 Nanoclusters for Enhanced Methanol Electrooxidation.

The study presents the first example of an adsorbed carbon monoxide (CO) enabled self-terminated Au-grafting on triphenylphosphine (PPh3) stabilized Pt6 nanoclusters (NCs) (Pt6 (PPh3)4Cl5 NCs or Pt6 NCs). Adsorbed PPh3 ligands weaken the Pt-CO bond enabling the self-terminated Au-grafting on Pt6 NCs. The Au-grafted Pt6 NCs exhibit enhanced methanol electrooxidation (MOR) in acidic solutions. The surface is composed of a PtAu ensemble exhibiting enhanced MOR and CO tolerance due to the synergistic interaction of Pt with Au and PPh3. The hydrogen underpotential deposition (H-UPD) signal from a CO-covered surface reveals the existence of free-Pt sites on the PtAu ensemble causing higher MOR reactivity. The Au and PPh3 ensure electrocatalytic activity of the NCs, depriving of them at anodic potentials results in "a death-valley" trend.

(111) Facet-oriented Cu2Mg Intermetallic Compound with Cu3-Mg Sites for CO2 Electroreduction to Ethanol with Industrial Current Density.

The efficient ethanol electrosynthesis from CO2 is challenging with low selectivity at high CO2 electrolysis rates, due to the competition with H2 and other reduction products. Copper-based bimetallic electrocatalysts are potential candidates for the CO2-to-ethanol conversion, but the secondary metal has mainly been focused on active components (such as Ag, Sn) for CO2 electroreduction, which also promote selectivity of ethylene or other reduction products rather than ethanol. Limited attention has been given to alkali-earth metals due to their inherently active chemical property. Herein, we rationally synthesized a (111) facet-oriented nano Cu2Mg (designated as Cu2Mg(111)) intermetallic compound with high-density ordered Cu3-Mg sites. The in situ Raman spectroscopy and density function theory calculations revealed that the Cu3 - --Mg- + active sites allowed to increase *CO surface coverage, decrease reaction energy for *CO-CO coupling, and stabilize *CHCHOH intermediates, thus promoting the ethanol formation pathway. The Cu2Mg(111) catalyst exhibited a high FEC2H5OH of 76.2±4.8 % at 600 mA⋅cm-2, and a peak value of |jC2H5OH| of 720±34 mA⋅cm-2, almost 4 times of that using conventional Cu2Mg with (311) facets, comparable to the best reported values for the CO2-to-ethanol electroreduction.

(111) Facet‐oriented Cu2Mg Intermetallic Compound with Cu3‐Mg Sites for CO2 Electroreduction to Ethanol with Industrial Current Density

AbstractThe efficient ethanol electrosynthesis from CO2 is challenging with low selectivity at high CO2 electrolysis rates, due to the competition with H2 and other reduction products. Copper‐based bimetallic electrocatalysts are potential candidates for the CO2‐to‐ethanol conversion, but the secondary metal has mainly been focused on active components (such as Ag, Sn) for CO2 electroreduction, which also promote selectivity of ethylene or other reduction products rather than ethanol. Limited attention has been given to alkali‐earth metals due to their inherently active chemical property. Herein, we rationally synthesized a (111) facet‐oriented nano Cu2Mg (designated as Cu2Mg(111)) intermetallic compound with high‐density ordered Cu3‐Mg sites. The in situ Raman spectroscopy and density function theory calculations revealed that the Cu3− −‐Mg− + active sites allowed to increase *CO surface coverage, decrease reaction energy for *CO−CO coupling, and stabilize *CHCHOH intermediates, thus promoting the ethanol formation pathway. The Cu2Mg(111) catalyst exhibited a high FEC2H5OH of 76.2±4.8 % at 600 mA⋅cm−2, and a peak value of |jC2H5OH| of 720±34 mA⋅cm−2, almost 4 times of that using conventional Cu2Mg with (311) facets, comparable to the best reported values for the CO2‐to‐ethanol electroreduction.

Computational demystification of iron carbonyls formation under syngas environment

Iron pentacarbonyl (IPC) gas forms upon the reaction of carbon monoxide with Fe containing metallic surfaces under gas reforming conditions. IPC formation can sometimes reach alarming levels that cause metal loss, pipeline thinning corrosion, catalyst poisoning, and contamination of sensitive industrial equipment. In this work, we demystify using multiscale computational modeling the mechanism of Iron pentacarbonyl formation: Density functional theory (DFT) is used to explore various catalytic reactions that involve a Fe adatom reacting with adsorbed carbon monoxide. Our calculated carbonyls desorption barriers on a perfect and clean Fe surface are too high to allow the carbonyls to form then desorb at temperatures <500 K at the rates reported experimentally. Most importantly, our calculations indicate that a high CO surface coverage, in addition to the presence of Fe adatoms, favors carbonyl formation and its desorption towards the flowing gas medium. Using insights extracted from ab initio molecular dynamics simulations, we propose that the most plausible IPC formation mechanism consists of: (1) on surface reactions of adsorbed CO molecules with an Fe adatom to form iron tricarbonyl (Fe(CO)3*) molecules; (2) an adsorbate assisted movement of iron tricarbonyl on top of the CO adlayer; and (3) the interaction of iron tricarbonyl with CO molecules from the gaseous medium eventually leading to iron adatom removal as Fe(CO)5 gas.

Restructuring multi-phase interfaces from Cu-based metal-organic frameworks for selective electroreduction of CO2 to C2H4.

Multi-phase interfaces are promising for surmounting the energy barriers of electrochemical CO2 reduction involving multiple electron transfer steps, but challenges still remain in constructing interfacial micro-structures and unraveling their dynamic changes and working mechanism. Herein, highly active Ag/Cu/Cu2O heterostructures are in situ electrochemically restructured from Ag-incorporating HKUST-1, a Cu-based metal-organic framework (MOF), and accomplish efficient CO2-to-C2H4 conversion with a high faradaic efficiency (57.2% at -1.3 V vs. RHE) and satisfactory stability in flow cells, performing among the best of recently reported MOFs and their derivatives. The combination of in/ex situ characterizations and theoretical calculations reveals that Ag plays a crucial role in stabilizing Cu(i) and increasing the CO surface coverage, while the active Cu/Cu2O interfaces significantly reduce the energy barrier of C-C coupling toward the boosted ethylene production. This work not only proves MOFs as feasible precursors to derive efficient electrocatalysts on site, but also provides in-depth understanding on the working interfaces at an atomic level.

How Temperature Affects the Selectivity of the Electrochemical CO2 Reduction on Copper.

Copper is a unique catalyst for the electrochemical CO2 reduction reaction (CO2RR) as it can produce multi-carbon products, such as ethylene and propanol. As practical electrolyzers will likely operate at elevated temperatures, the effect of reaction temperature on the product distribution and activity of CO2RR on copper is important to elucidate. In this study, we have performed electrolysis experiments at different reaction temperatures and potentials. We show that there are two distinct temperature regimes. From 18 up to ∼48 °C, C2+ products are produced with higher Faradaic efficiency, while methane and formic acid selectivity decreases and hydrogen selectivity stays approximately constant. From 48 to 70 °C, it was found that HER dominates and the activity of CO2RR decreases. Moreover, the CO2RR products produced in this higher temperature range are mainly the C1 products, namely, CO and HCOOH. We argue that CO surface coverage, local pH, and kinetics play an important role in the lower-temperature regime, while the second regime appears most likely to be related to structural changes in the copper surface.

Elucidating the Roles of Nafion/Solvent Formulations in Copper-Catalyzed CO2 Electrolysis.

Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO2 reduction, but its impact on the performance of CO2 electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO2 electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO2 electrolysis performance, resulting in variations of C2+ product Faradaic efficiency (FE) by >3×, with C2+ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO2 reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO2/H2O concentration ratio and promoting high CO surface coverage to facilitate C2+ production in long-term CO2 electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO2 reduction systems.

Investigation of gas transport resistance in fuel cell catalyst layers via hydrogen limiting current measurements of CO-covered catalyst surfaces

The efficiency of fuel cell operation at high power strongly affects the total cost of ownership of fuel cell vehicles. Gas transport resistance at the cathode strongly suppresses efficiency during high-power operation. The resistance comprises Knudsen diffusion resistance in pores formed among the primary carbon aggregates and local gas transport resistance. The former is independent of the electrochemically active Pt surface area per geometrical electrode area (roughness factor: RF). The latter is proportional to the inverse of RF. Separate identification of these resistances in actual catalyst layers remains challenging because of experimental limitations. Herein, we propose a method that can identify the RF-dependent and RF-independent resistances separately using a single sample. RF can be controlled by partially covering the Pt surface by CO. The gas transport resistance can be ascertained from the hydrogen limiting current. Measuring the hydrogen limiting current at controlled CO coverages allows separate identification of the resistances. The RF-dependent resistance is separable further into ionomer/Pt interfacial resistance and permeation resistance through the ionomer thin film by combining the obtained ionomer-to-carbon weight ratio dependence of each resistance component with structural and electrochemical analyses. The separate resistances show the ionomer/Pt interfacial resistance to be dominant in the catalyst layer.

Pt-Based Nanoreactors Derived from ZIF-67 Nanocubes on Al2O3 Films for Low-Temperature CO Oxidation

In the field of catalysis, nanoreactors with spatially restricted effects play a critical role to enhance the production efficiency of a variety of reactions. In this contribution, we used ZIF-67 nanocubes (size of 140 nm) as a template and conformally deposited Al2O3 thin films by the atomic layer deposition (ALD) technique, wherein Pt nanoparticles (Pt NPs) as active sites were loaded on either the inside or outside of the Al2O3 films. Notably, it was found that the water treatment of the nanoreactors was critical to the enhanced catalytic CO oxidation performance with substantially decreased T100 (temperature to achieve 100% conversion). Various characterization results indicated the successful synthesis of yolk–shell structured ZIF-67/Pt@10Al2O3-W catalyst with excellent CO oxidation activity and good long-term stability. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy was used to explore the catalytic oxidation process, and the results revealed that the reason for the superior CO catalytic activity of the designed nanoreactors was ascribed to the moderate CO surface coverage provided by the Al2O3 films and the activated oxygen species. In summary, this work provides some insights into the preparation of metal–organic framework-based nanoreactors using ALD-assisted methods.

Grain Boundary-Derived Cu+ /Cu0 Interfaces in CuO Nanosheets for Low Overpotential Carbon Dioxide Electroreduction to Ethylene.

Electrochemical CO2 reduction reaction can be used to produce value‐added hydrocarbon fuels and chemicals by coupling with clean electrical energy. However, highly active, selective, and energy‐efficient CO2 conversion to multicarbon hydrocarbons, such as C2H4, remains challenging because of the lack of efficient catalysts. Herein, an ultrasonication‐assisted electrodeposition strategy to synthesize CuO nanosheets for low‐overpotential CO2 electroreduction to C2H4 is reported. A high C2H4 Faradaic efficiency of 62.5% is achieved over the CuO nanosheets at a small potential of −0.52 V versus a reversible hydrogen electrode, corresponding to a record high half‐cell cathodic energy efficiency of 41%. The selectivity toward C2H4 is maintained for over 60 h of continuous operation. The CuO nanosheets are prone to in situ restructuring during CO2 reduction, forming abundant grain boundaries (GBs). Stable Cu+/Cu0 interfaces are derived from the low‐coordinated Cu atoms in the reconstructed GB regions and act as highly active sites for CO2 reduction at low overpotentials. In situ Raman spectroscopic analysis and density functional theory computation reveal that the Cu+/Cu0 interfaces offer high *CO surface coverage and lower the activation energy barrier for *CO dimerization, which, in synergy, facilitates CO2 reduction to C2H4 at low overpotentials.

On the link between CO surface coverage and selectivity to CH4 during CO2 hydrogenation over supported cobalt catalysts

An operando IR study was carried out to investigate the loss of selectivity to higher hydrocarbons observed over cobalt-based Fischer-Tropsch catalysts when CO is replaced with CO2. Our data unambiguously show that the structure and oxidation state of the surface of metallic cobalt remains the same whether CO2 or CO are cofed with H2. The selectivity to CH4 appears to increase monotonously with decreasing surface coverage of adsorbed CO. The switch from Fischer-Tropsch synthesis to methanation using CO2-rich feeds over metallic cobalt can therefore be assigned to surface coverage effects only, assuming that most, if not all, hydrocarbons products are formed on metallic cobalt.

Ruthenium Icosahedra and Ultrathin Platelets: The Role of Surface Chemistry on the Nanoparticle Structure

The control of the crystalline structure and shape (crystal habit) of nanoparticles (NPs) is the key to controlling their physical and chemical properties. Among the different metals, the crystallogenesis of ruthenium NPs has been less studied, and until recently, the Ru NP crystal structure and morphology have been considered as presenting less versatility than the face-centered cubic (fcc) metals of the platinum group. Here, we show that while the hydrogenation of [Ru(COD)(COT)] in solutions containing a long-chain amine (hexadecylamine, HDA) in large excess leads to isotropic NPs adopting the expected hexagonal close-packed (hcp) structure of bulk Ru, a long-chain carboxylic acid (lauric acid, LA) in large excess induces the formation of Ru nano-objects of two original structures: ultrathin platelets and icosahedra. The latter have never been produced so systematically by other methods. We show that carbon monoxide, produced in situ by the decarbonylation of lauric acid, plays a pivotal role in the stabilization of the Ru icosahedra. This result is supported by density functional theory (DFT) calculations, which show that above a critical surface coverage of CO, small Ru icosahedra are more stable than the Ru bipyramid polyhedra crystallizing in the hcp structure. Thus, in situ production of CO results in a competition between icosahedral and hcp seeds, which explains the mixture of icosahedra and ultrathin platelets. Another effect of the large excess of lauric acid is the stabilization of the ruthenium precursors in solution, limiting the nucleation extent and slowing down the NP growth. While the growth of the icosahedral seeds is limited because of the structural strains, the growth of the hcp seeds preferentially develops the (0001) facets, leading to ultrathin platelets and threefold stars.

C−C Coupling Is Unlikely to Be the Rate‐Determining Step in the Formation of C2+ Products in the Copper‐Catalyzed Electrochemical Reduction of CO

AbstractThe identity of the rate‐determining step (RDS) in the electrochemical CO reduction reaction (CORR) on Cu catalysts remains unresolved because: 1) the presence of mass transport limitation of CO; and 2) the absence of quantitative correlation between CO partial pressure (pCO) and surface CO coverage. In this work, we determined CO adsorption isotherms on Cu in a broad pH range of 7.2–12.9. Together with electrokinetic data, we demonstrate that the reaction orders of adsorbed CO at pCO &lt;0.4 and &gt;0.6 atm are 1st and 0th, respectively, for multi‐carbon (C2+) products on three Cu catalysts. These results indicate that the C−C coupling is unlikely to be the RDS in the formation of C2+ products in the CORR. We propose that the hydrogenation of CO with adsorbed water is the RDS, and the site competition between CO and water leads to the observed transition of the CO reaction order.

C-C Coupling Is Unlikely to Be the Rate-Determining Step in the Formation of C2+ Products in the Copper-Catalyzed Electrochemical Reduction of CO.

The identity of the rate-determining step (RDS) in the electrochemical CO reduction reaction (CORR) on Cu catalysts remains unresolved because: 1) the presence of mass transport limitation of CO; and 2) the absence of quantitative correlation between CO partial pressure (pCO ) and surface CO coverage. In this work, we determined CO adsorption isotherms on Cu in a broad pH range of 7.2-12.9. Together with electrokinetic data, we demonstrate that the reaction orders of adsorbed CO at pCO <0.4 and >0.6 atm are 1st and 0th , respectively, for multi-carbon (C2+ ) products on three Cu catalysts. These results indicate that the C-C coupling is unlikely to be the RDS in the formation of C2+ products in the CORR. We propose that the hydrogenation of CO with adsorbed water is the RDS, and the site competition between CO and water leads to the observed transition of the CO reaction order.

Stroboscopic operando spectroscopy of the dynamics in heterogeneous catalysis by event-averaging

Heterogeneous catalyst surfaces are dynamic entities that respond rapidly to changes in their local gas environment, and the dynamics of the response is a decisive factor for the catalysts’ action and activity. Few probes are able to map catalyst structure and local gas environment simultaneously under reaction conditions at the timescales of the dynamic changes. Here we use the CO oxidation reaction and a Pd(100) model catalyst to demonstrate how such studies can be performed by time-resolved ambient pressure photoelectron spectroscopy. Central elements of the method are cyclic gas pulsing and software-based event-averaging by image recognition of spectral features. A key finding is that at 3.2 mbar total pressure a metallic, predominantly CO-covered metallic surface turns highly active for a few seconds once the O2:CO ratio becomes high enough to lift the CO poisoning effect before mass transport limitations triggers formation of a √5 oxide.

H2 production by methane steam reforming over Rh/Al2O3 catalyst packed in Cu foams: A strategy for the kinetic investigation in concentrated conditions

The concept of Rh/Al2O3 catalyst pellets packed in highly conductive copper foams has been successfully tested in methane steam reforming showing the beneficial effects of thermal conductivity on the obtainment of gradient-less radial temperature profiles. In this work, the same concept is proposed as a strategy for the lab-scale kinetic investigation under concentrated conditions at ambient pressure; thanks to the homogeneous heating of the catalyst mass across the reactor section and the measurement of axial temperature profiles, well-controlled temperature conditions are obtained, and the experimental investigation can be extended to usually unfeasible conditions of high reactant concentrations, overcoming the typical challenges for the kinetic study. Here, steam reforming experiments were performed with CH4 and H2O feed molar fractions in the ranges of 10−20 % and 40–90 %, respectively. The co-feed of CO and H2 was also investigated. A kinetic scheme was developed that substantially confirmed the main results of previous kinetic investigations in annular micro-reactor, performed under diluted conditions; in particular, the first order dependence of the rate of steam reforming on methane partial pressure, the independence from H2O partial pressure, and the important inhibiting effect of CO were confirmed. The independence of the reaction rate from the H2 co-feed was here demonstrated for the first time. The new experimental campaign allowed to identify more clearly the kinetic dependencies of the water gas shift reaction, positively influenced by H2O partial pressure but scarcely affected by CO partial pressure, which could be also explained based on the inhibiting effect of surface CO coverage. Parameter estimates were obtained by model fit over a wide temperature range (400−850 °C), conveying robustness to the proposed kinetic scheme for future reactor design applications.

Optimizing CO Coverage on Rough Copper Electrodes: Effect of the Partial Pressure of CO and Electrolyte Anions (pH) on Selectivity toward Ethylene

The conversion of the initial intermediate CO in the electrochemical reduction reaction of CO2 on the surface of oxide-derived Cu electrodes has been investigated as a function of partial pressure and pH, manipulated by the composition of the electrolyte. We show that in inert gas, an increase in partial pressure of CO results in a continuous increase in Faradaic efficiency (FE) for ethylene, at various potentials ranging from −0.7 to −1.1 V versus RHE, with the highest FE of ∼28% obtained using 1 bar CO at −0.8 V. When the partial pressure of CO is increased in a mixture of CO and CO2, an optimum in the ethylene FE was found for the partial pressure of CO in the range from 0.5 bar (at −1.1 V, FE is ∼45%) to 0.8 bar (at −0.9 V, FE is ∼35%). At lower negative potentials (−0.8 to −0.7 V), the presence of CO2 has negligible influence, and similar data to reduction of CO in inert gas were obtained. Variation of the anion in solution (0.1 M concentration) shows that the optimized FE toward ethylene increases from 5.2% in KH2PO4 to 43.2% in KOH. The observed differences in selectivity are attributed to anion buffering capacity and the associated local pH near the surface of the electrode. Using in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), it was determined that the CO coverage increases as a function of increasing pH, confirming that CO coverage and pH correlate. Collectively, the data herein outline the critical role of reactant partial pressures and the significant effect of anion composition (pH) on the surface coverage of CO and concomitant selectivity in electrochemical reduction of CO2 to ethylene.

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Summary Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation.

Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO2 Adsorption Kinetics.

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO2 electroreduction on Au. We use in-situ, plasmon-enhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potential-dependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO2 adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO2 reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.

CuO/ZnO/C electrocatalysts for CO2-to-C2+ products conversion with high yield: On the effect of geometric structure and composition

The comprehensive application of electrochemical CO2 conversion to multi-carbon hydrocarbons and oxygenates is limited by the low selectivity and activity of copper-derived catalysts. Here, we report co-precipitated CuO/ZnO/C catalysts directing cascade catalysis to enhance the yield of the C2+ products. The reduced ZnO nanoparticles supply extra CO to increase the *CO surface coverage on the reduced CuO nanoparticles, which boosts the CC coupling reaction kinetics on the reduced CuO surface. The geometric arrangement and composition of phase-separated CuO and ZnO nanoparticles are two vital factors influencing the CO utilization efficiency. The optimum CuO/ZnO/C catalyst exhibits a maximum Faradic efficiency of 74.0 % for overall C2+ products and 50.9 % for C2H4 at -0.75 V vs. reversible hydrogen electrode, at which the total current density reaches 367 mA cm−2.