Local CO Coverage Research Articles

Enhanced Local CO Coverage on Cu Quantum Dots for Boosting Electrocatalytic CO2 Reduction to Ethylene

AbstractEthylene (C2H4) electrosynthesis from the electrocatalytic CO2 reduction process holds enormous potential applications in industrial production. However, the sluggish kinetics of C─C coupling often result in low yield and poor selectivity for C2H4 production. Herein, the performance of Cu catalysts of varying sizes is investigated, prepared via a cryo‐mediated liquid phase exfoliation technique, for the electrochemical CO2 reduction to C2H4. The activity and selectivity of C2H4 gradually increase as the size of the Cu catalysts decreases from tens of nanometers to a few nanometers. Impressively, the 5 nm Cu quantum dots (Cu‐5) achieve a maximum C2H4 Faradaic efficiency (FE) of 81.5% and a half‐cell cathodic energy efficiency (CEE) of 42.2% with a large partial current density of 1.1 A cm−2 at −0.93 V versus the reversible hydrogen electrode. Structural characterization and in situ spectroscopic analysis reveal that the Cu‐5 quantum dots, dominated by the (100) facet, provide an abundance of active sites that enhance CO2 adsorption and activation, promoting the formation of *CO intermediates. The accumulation of *CO intermediates on the Cu active sites facilitates the CO‐CHO coupling reaction, thus enhancing the C2H4 production rate.

Cascade Dual Sites Modulate Local CO Coverage and Hydrogen-Binding Strength to Boost CO2 Electroreduction to Ethylene.

Rationally modulating the binding strength of reaction intermediates on surface sites of copper-based catalysts could facilitate C-C coupling to generate multicarbon products in an electrochemical CO2 reduction reaction. Herein, theoretical calculations reveal that cascade Ag-Cu dual sites could synergistically increase local CO coverage and lower the kinetic barrier for CO protonation, leading to enhanced asymmetric C-C coupling to generate C2H4. As a proof of concept, the Cu3N-Ag nanocubes (NCs) with Ag located in partial Cu sites and a Cu3N unit center are successfully synthesized. The Faraday efficiency and partial current density of C2H4 over Cu3N-Ag NCs are 7.8 and 9.0 times those of Cu3N NCs, respectively. In situ spectroscopies combined with theoretical calculations confirm that Ag sites produce CO and Cu sites promote asymmetric C-C coupling to *COCHO, significantly enhancing the generation of C2H4. Our work provides new insights into the cascade catalysis strategy at the atomic scale for boosting CO2 to multicarbon products.

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.

CO on Supported Cu Nanoclusters: Coverage and Finite Size Contributions to the Formation of Carbide via the Boudouard Process

The interaction of carbon monoxide with an ordered array of copper nanoclusters was investigated under ultrahigh vacuum conditions by means of in situ X-ray photoelectron spectroscopy in combination with density functional theory calculations. The Cu clusters were supported on an alumina template grown on the Ni3Al(111) termination. Adsorption and dissociation of carbon monoxide occur at the copper clusters, yielding accumulation of carbidic carbon at the metal particles through the Boudouard process. The involved mechanisms are investigated at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and of local CO coverage. It is found that the high coverage of CO at the cluster surface, which considerably exceeds that achievable on single crystal surfaces, facilitates the metal restructuring and the reaction, yielding carbon incorporation into the bulk of the particles.

Role of surface defect sites: from Pt model surfaces to shape-controlled nanoparticles

In the present paper, preferentially oriented (111) Pt nanoparticles (mostly octahedral and tetrahedral, namely {111}Pt nanoparticles) have been characterized and compared with a Pt(554) single-crystal electrode as their voltammetric features are quite similar in 0.5 M H2SO4. The anion and Bi adsorption behaviours suggest that the {111}Pt nanoparticles contain relatively wide hexagonal domains and also isolated sites which could adsorb solely hydrogen. Bi step decoration has been successfully extended to modify the defects of {111}Pt nanoparticles without blocking terrace sites. CO charge displacement has been applied to determine the potential of zero total charge (pztc) of non-decorated and Bi decorated surfaces. It has found that the positive shift of pztc on defect-decorated {111}Pt nanoparticles is not so significant in comparison with that of Pt(554) due to the relative short mean length of (111) domains on the {111}Pt nanoparticles. CO stripping demonstrates that {111}Pt nanoparticles exhibit higher reactivity toward CO oxidation. This reflects the role of the defect sites in nanoparticles, evidenced by the disappearance of the “pre-wave” in the stripping voltammogram once the defects were blocked by Bi. The stripping peaks shift to higher potential on Bi decorated surfaces, indicating the active role of both steps and defects for CO oxidation. By comparing the CO stripping charge and the change in hydrogen adsorption charge of surfaces with and without Bi decoration, including reasonable deconvolution, the local CO coverage on defect and terrace sites were obtained for the first time for the {111}Pt nanoparticles, and the results are in good agreement with those obtained on Pt(554). Chronoamperometry studies show tailing in all current–time transients of CO oxidation on all surfaces studied. The kinetics of CO oxidation can be satisfactorily simulated by a modified Langmuir–Hinshelwood model, demonstrating that CO oxidation on all studied surfaces follows the same mechanism.

Methanol Oxidation on Pt(111)/Ru in Alkaline Media

Dehydrogenation and oxidation of methanol has been studied on well-ordered Pt(111), vicinal Pt(111), and Pt(111)/Ru in alkaline media. Multiple mechanisms of promotion have been identified. Steps with (110) orientation promote methanol dehydrogenation through a geometric effect, while they promote CO oxidation through a change in electronic structure. Adsorbed Ru promotes methanol dehydrogenation through two distinct electronic (ligand) effects: it increases the intrinsic activity of nearby Pt sites for methanol dehydrogenation, and it decreases the local CO coverage at the active sites even when the overall CO coverage is high. Ru also promotes methanol oxidation through a ligand effect on CO oxidation. The bifunctional effect of Ru is concluded to be much weaker in alkaline media than in acidic media. STM imaging confirms the stability of adsorbed Ru even during extended potential cycling and oxidizing treatments, demonstrating the promising characteristics of Pt/Ru surfaces for fuel cells.

Surface Chemistry on Bimetallic Alloy Surfaces: Adsorption of Anions and Oxidation of CO on Pt3Sn(111)

The microscopic structure of the Pt(3)Sn(111) surface in an electrochemical environment has been studied by a combination of ex situ low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) and in situ surface X-ray scattering (SXS) and Fourier transform infrared (FTIR) spectroscopy. In ultrahigh vacuum (UHV) the clean-annealed surface produces a p(2 x 2) LEED pattern consistent with the surface composition, determined by LEIS, of 25 at. % Sn. SXS results show that the p(2 x 2) structure can be "transferred" from UHV into 0.5 M H(2)SO(4) and that the surface structure remains stable from 0.05 to 0.8 V. At 0.05 V the expansion of Pt surface atoms, ca. +2% from the bulk lattice spacing, is induced by adsorption of underpotential-deposited (UPD) hydrogen. At 0.5 V, where Pt atoms are covered by (bi)sulfate anions, the topmost layer is contracted relative to 0.05 V, although Sn atoms expand significantly, ca. 8.5%. The p(2 x 2) structure is stable even in solutions containing CO. In contrast to the Pt(111)-CO system, no ordered structures of CO are formed on the Pt(3)Sn(111) surface and the topmost layer expands relatively little (ca. 1.5%) from the bulk lattice spacing upon the adsorption of CO. The binding site geometry of CO on Pt(3)Sn(111) is determined by FTIR. In contrast to the near invariant band shape of a-top CO on Pt(111), changes in band morphology (splitting of the band) and vibrational properties (increase in the frequency mode) are clearly visible on the Pt(3)Sn(111) surface. To explain the line shape of the CO bands, we suggest that in addition to alloying effects other factors, such as intermolecular repulsion between coadsorbed CO and OH species, are controlling segregation of CO into cluster domains where the local CO coverage is different from the coverage expected for the CO-CO interaction on an unmodified Pt(111) surface.

The structure, growth and reactivity of electrodeposited Ru/Au(111) surfaces

In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru:Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H2SO4 solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (\0.1 V), (defective) monolayer (] 0.1 V), and multilayer films (B 0.1 V). At potentials 0.6 V irreversible formation of Ru oxide:hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at :0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film. © 2001 Elsevier Science B.V. All rights reserved.

A molecular beam study of nonlinearity in the CO-induced surface restructuring of Ir{100}

The kinetics of CO chemisorption on both the (1×5) and (1×1) surfaces of Ir{100}, including the CO-induced surface restructuring process, have been studied by measuring the sticking probability as a function of the surface temperature and beam flux. Due to competition between desorption from the (1×5) phase and growth of (1×1) islands, the sticking probability on the initial (1×5) surface is strongly flux-dependent at surface temperatures Ts in the range 480⩽Ts⩽510 K. It is shown that this is due to a strongly nonlinear dependence of the (1×1) growth rate on the local CO coverage on the (1×5) substrate, with an apparent reaction order of around 5. Desorption energies and pre-exponentials of desorption for CO from both the (1×1) and (1×5) surfaces have been determined by means of a modified lifetime measurement technique. Equilibrium coverages as well as isothermal desorption rates of CO were determined for both surface phases. The zero coverage desorption energy of CO from the (1×1) substrate is 196±5 kJ/mol and from the (1×5) surface it is around 150 kJ/mol. This difference in adsorption energies is the driving force for the CO-induced (1×5) to (1×1) phase transition. TEAS data show that the local CO coverage on the growing (1×1) islands during the phase transformation is 0.5 ML.

Theoretical Inquiry into the Microscopic Origins of the Oscillatory CO Oxidation Reaction on Pt{100}

An improved model is presented which simulates temporal behavior in the CO + O2 reaction on the Pt{100} surface. The model is based on an experimentally determined strongly nonlinear power law for (1 × 1)-CO island growth rate from the hexagonal (hex) phase with an apparent reaction order of about 4 in the local CO coverage on the hex phase. The power law describes the phase transition from the hexagonal phase of the reconstructed Pt surface to the (1 × 1) phase. Rate parameters from recent adsorption and desorption experiments are used in the modeling. The variations of all adsorbate coverages, the reaction rates, and the surface phase are monitored to provide extensive information about the reaction. New insight into the mechanisms driving the experimentally observed oscillations is gained, and an interpretation of the actual dynamics of the underlying surface processes is also given.

Modelling temporal kinetic oscillations for CO oxidation on Pt/s(100/s). The (1 × 1)-CO island growth rate power law model

An improved model which accurately reproduces the observed kinetic oscillations for CO oxidation on Pt/s(100/s) is presented. It incorporates a strongly non-linear power law for the (1 × 1)-CO island growth rate from the stable hex phase with an apparent reaction order of about four in the local CO coverage on the hex phase. This experimentally determined power law replaces the formerly assumed first-order process. Calculations for the CO+C 2 system reveal that the oscillatory behavior is very sensitive to the assumed value for the reaction order; values less than three do not produce the characteristic oscillatory behaviour. Hysteresis between two rate branches is demonstrated in calculations of the steady states as a function of (1 × 1) surface area; these calculations provide a ready means of interpreting the kinetic oscillations.

NONLINEAR EFFECTS IN ADSORPTION AND REACTION DYNAMICS AT SURFACES: KINETIC OSCILLATIONS UNDER SCRUTINY

A number of catalytic reactions exhibit complex dynamical behavior, including kinetic oscillations with both temporal and spatial characteristics. Attention has been focused on the CO+ NO and CO+O2 reactions on Pt{100}. Recent molecular beam studies conducted at Cambridge on the interaction of CO with Pt{100} have provided the basis for a new mechanism for oscillatory behavior. The rate of growth of (1×1) islands on the quasihexagonal reconstructed phase of Pt{100} is strongly nonlinear with respect to the local CO coverage on the hex-R phase at temperatures between 380 and 410 K, with a reaction order of 4. This strong nonlinearity results in a flux-dependent sticking probability for CO at this temperature, and detailed modeling also reproduces the major features in the oscillatory behavior observed for the CO+NO reaction. An important effect of the nonlinearity in both the CO+NO and CO+O2 oscillatory behavior at high temperatures is the maintenance of in-phase behavior across the surface during oscillations.

Electrochemical and vacuum behavior of carbon monoxide and lead coadsorbed on platinum(111)

The electrochemical and vacuum behavior of coadsorbed CO and Pb on Pt(111) was examined in situ by cyclic and linear sweep voltammetry and ex situ by Auger electron spectroscopy and thermal desorption spectroscopy (TDS) in ultrahigh vacuum (UHV). Lead was deposited electrochemically from 0.1 M HClO 4 with 1 mM Pb 2+, and CO was adsorbed by either solution dosing in situ or vapor dosing in UHV. Coadsorbed CO alters the Pb surface redox reaction Pb ad 2+ Pb ad⇔ 0,reducing its potential from 0.65 V/RHE in 0.1 M HClO 4 without CO to 0.52 V with CO. The CO + Pb/Pt(111) system exhibits dynamic emersion with a constant emersion potential of about 0.5 V established by the CO-shifted Pb surface redox reaction. The electro-oxidation potential of CO, which is in the range 0.7–0.8 V in the absence of Pb, is largely unaffected by small and moderate coverages of Pb. A near-saturated monolayer of Pb (θ pb = 0.60 expressed as Pb atoms per Pt surface atom) substantially reduces the over potential for CO oxidation, with the reaction occurring over the range 0.3–0.6 V. In agreement with earlier studies, substantially more CO can be adsorbed by solution dosing than by vapor dosing. The maximum local CO coverage for solution dosing is approximately θ CO,loc = 0.7 (molecules per Pt surface atom) for Pb coverages ranging from zero to 0.4. For vapor dosing the maximum local CO coverage is θ CO,loc = 0.5 independent of Pb coverage. Either species, when present as a saturated monolayer, blocks adsorption of the other, and Pb at lower coverages acts as a site blocker towards CO. The activation energy for CO desorption was measured with TDS and fitted to a three-parameter model over the entire range of allowed Pb and CO coverages. This model shows that coadsorbed Pb destabilizes CO by reducing its activation energy for desorption by up to 15%.

A molecular beam study of the CO-induced surface phase transition on Pt{100}

The CO-induced hex→(1×1) surface phase transition on Pt{100} is driven by the higher adsorption heat of CO on the (1×1) phase than on the hex phase and occurs by the sequential steps of CO adsorption on the hex phase and growth of islands of CO trapped on the (1×1) phase. We have investigated the dynamics of CO chemisorption on both the rotated hex (hex-R) and (1×1) surfaces by measuring the sticking probability as a function of surface temperature, beam energy, and beam incidence angle using the King and Wells beam reflectivity method. For both clean surfaces, the adsorption is nonactivated and there is no evidence for adsorption via an intrinsic precursor. However, at finite CO coverages, there is an additional adsorption pathway involving an extrinsic physisorbed precursor. At surface temperatures above 350 K, the net sticking probability on the initial hex-R surface at finite CO coverages is less than below 350 K due to CO desorption, primarily from the hex-R phase, competing with island growth. We have developed a technique to measure the local CO coverage on the hex-R phase during the CO-induced hex-R→(1×1) phase transformation. The island growth rate has a power law dependence on the local CO coverage on the hex-R phase with an apparent reaction order of 4.5±0.4. These kinetics manifest themselves as a strongly flux-dependent net sticking probability. The absence of a decrease in the local CO coverage on the hex-R phase after the onset of island growth is unexpected considering the nucleation-and-growth process, and two possible microscopic mechanisms for the surface phase transition are proposed.

Surface restructuring dynamics in CO adsorption, desorption, and reaction with NO on Pt{100}

A detailed experimental molecular beam study of the CO-induced surface phase transition on Pt{100}, from the clean surface hex-R0.7° structure of the bulk termination (1 × 1) structure, has recently been completed in our laboratory. In the present work a stepwise mechanistic scheme is presented which accounts for the observed sticking probability dependence on both coverage and substrate temperature, including a strong flux dependence in the net sticking probability, and the behaviour of the desorption spectra. The shift in the highest-temperature peak to lower temperatures with increasing coverage is reproduced, together with zero order desorption kinetics. The model is based on the observed dependence of the (1 × 1) island growth rate from the hex-R phase with an apparent reaction order of 4.1 in the local CO coverage on the hex-R phase, and reproduces the observed hysteresis in the surface phase transition when Pt{100} is heated or cooled in CO. Kinetic oscillations in the reaction between CO and NO to form CO 2 and N 2 are reproduced by the model within two temperature regimes, in close agreement with experiment. In the higher temperature regime the oscillations are assigned to a new mechanism involving the hex↓(1 × 1) phase transition. As these transitions proceed surface defects are generated, and these defects are shown to play an important role in the time scale of the oscillations. All the major features of adsorbate-induced restructuring, desorption and the CO + NO oscillatory reaction are therefore successfully reproduced by this single mechanistic scheme.

Nonlinear island growth dynamics in adsorbate-induced restructuring of quasihexagonal reconstructed Pt{100} by CO.

We have made the first dynamic measurement of the (1\ifmmode\times\else\texttimes\fi{}1)-CO island growth rate and the simultaneous CO coverage on the quasihexagonal reconstructed (hex-R) phase during the CO-induced hex-R\ensuremath{\rightarrow}(1\ifmmode\times\else\texttimes\fi{}1) phase transformation on Pt{100}. The island growth rate is described by a strongly nonlinear power law with respect to the local CO coverage on the hex-R phase at surface temperatures between 380 and 410 K, with an apparent reaction order of 4.5\ifmmode\pm\else\textpm\fi{}0.4. These kinetics manifest themselves as a strongly flux dependent net sticking probability.

Modelling the CO-induced surface phase transition on Pt{100}: implications for kinetic oscillations

A recent molecular beam study of the CO-induced hex →(1 × 1) surface phase transition on Pt{100} showed that the rate of phase transformation has a high reaction order (ca. 4.1) with respect to the local CO coverage on the rotated hexagonal (hex-R) phase. This result is used to formulate a new kinetic model for the CO + Pt{100} system. It is used to investigate features of CO temperature-programmed desorption spectra, and to predict the temperature and pressure dependence of the CO and NO coverages on the (1 × 1) phase during the hex →(1 × 1) surface phase transition (SPT) and under equilibrium conditions. At temperatures of around 450 K, where kinetic oscillations in the CO + NO reaction on Pt{100} are accompanied by oscillations in the relative area of the (1 × 1) phase, the adsorbate coverages on the (1 × 1) phase are too low to inhibit NO dissociation. An alternative mechanism for these oscillations is presented, involving the hex ↔(1 × 1) SPT and not the rate limitation of NO dissociation as the driving force. Our new model is used to develop a more detailed understanding of this new mechanism.

One-dimensional CO island formation in the coadsorption of H 2 and CO on the steps of Pt(112)

The interaction of coadsorbed CO and H 2 on the steps of Pt(112) [Pt(S)[3(111)×(001)]] was examined by digital electron-stimulated desorption ion angular distribution (ESDIAD), low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD). On the clean surface, CO and H 2 both adsorb preferentially on step sites at low coverages; CO at stop sites on the step edge and H 2, as atoms, in the step 4-fold sites. For pure CO adsorption, an ordered (2× n) structure of step CO is observed at 0.19 ML (atop step sites half filled), and at least two CO structural transformations occur as the step sites fill further. These structural transformations, due to COCO repulsions, involve the production of tilted CO species whose tilt directions are related to the local CO coverage in one-dimensional chains of CO on the step edges. The structural transformations described above for pure CO layers on the step edges can also be produced by coadsorption of hydrogen on the (2× n) step CO structure. The CO structural transformations in the coadsorbed system are driven by H adsorption, due to the formation of immiscible one-dimensional CO and H islands at the steps. Similar results are observed for low coverages of CO adsorbed on the H atom saturated surface.

The kinetics of CO dissociation on Ru(001): Time-resolved vibrational spectroscopy at elevated pressures

Time-resolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to measure the kinetics of CO dissociation on a Ru(001) surface at elevated pressures (10−3 to 10 Torr) and temperatures (500–700 K). The reaction of CO with Ru(001) is found to be a nonsteady state and results in CO disproportionation, i.e., 2CO→C+CO2. The decrease in total CO coverage follows first order kinetics and exhibits Arrhenius behavior with an activation energy of 20.6 kcal and a preexponential factor of 102 s−1. Comparison of the overall reaction rate with that of CO2 formation (O+CO→CO2) confirms that CO dissociation is the rate-limiting step in the disproportionation reaction. The in situ reaction rate constant exhibits a weak dependence on CO pressure (<first order). However, the determination of local CO coverages during reaction reveals a linear dependence of the dissociation rate with CO coverage. This confirms that the chemisorbed state of the molecule is a precursor to dissociation and that a high pressure is required to maintain a steady state surface coverage of CO at reaction temperature. In situ vibrational spectra demonstrate the formation of carbon islands under reaction conditions which prevent further CO adsorption and result in a decrease in total CO coverage at constant local CO coverage. Post-reaction spectroscopy confirms the formation of two-dimensional islands of carbon whose reactivity toward oxidation is found to be between that of amorphous carbon and three-dimensional graphite.

Infrared spectral studies of reactions of carbon monoxide and oxygen on [formula omitted

The reaction of oxygen gas with CO adsorbed on two samples of Pt SiO 2 with different particle sizes has been followed by direct monitoring of the infrared absorption near 2075 cm −1. In contrast to earlier reports the rate of CO 2 formation did not show a simple second-order dependence on CO coverage and this discrepancy is explained by improved spectrometer performance. During the course of reaction the peak frequency of the residual CO molecules remained almost constant indicating the maintenance of high local CO coverage throughout. This is interpretable in terms of island formation on individual particles with reaction only at the interface between islands. Evacuation halted oxidation even though the sample as a whole still contained both adsorbed oxygen and adsorbed CO. Reaction between them became impossible probably because individual particles contained only one adsorbed species. An accelerated rate was observed when oxygen was readmitted to a partially reacted system. Over a limited range the fall in θ CO was linear with ln(time) pointing to a steadily rising activation energy. Reaction of CO with an initially oxygen covered surface exhibited similar behavior whilst the presence of both gases led to maximal CO coverages below 100 °C even when oxygen was in excess. The origin of the increase in activation energy is believed to be the intrinsic inter- and intraparticulate heterogeneity of supported platinum.