Carbon Monoxide Surface Coverage Research Articles

Steering carbon dioxide reduction toward C–C coupling using copper electrodes modified with porous molecular films

Copper offers unique capability as catalyst for multicarbon compounds production in the electrochemical carbon dioxide reduction reaction. In lieu of conventional catalysis alloying with other elements, copper can be modified with organic molecules to regulate product distribution. Here, we systematically study to which extent the carbon dioxide reduction is affected by film thickness and porosity. On a polycrystalline copper electrode, immobilization of porous bipyridine-based films of varying thicknesses is shown to result in almost an order of magnitude enhancement of the intrinsic current density pertaining to ethylene formation while multicarbon products selectivity increases from 9.7 to 61.9%. In contrast, the total current density remains mostly unaffected by the modification once it is normalized with respect to the electrochemical active surface area. Supported by a microkinetic model, we propose that porous and thick films increase both local carbon monoxide partial pressure and the carbon monoxide surface coverage by retaining in situ generated carbon monoxide. This reroutes the reaction pathway toward multicarbon products by enhancing carbon–carbon coupling. Our study highlights the significance of customizing the molecular film structure to improve the selectivity of copper catalysts for carbon dioxide reduction reaction.

Operando characterisation of the impact of carbon monoxide on PEMFC performance using isotopic labelling and gas analysis

Impurities in hydrogen can have a detrimental effect on the performance of polymer electrolyte membrane fuel cells (PEMFCs) used in automotive applications. However, the establishment of reliable threshold limits for each impurity is hampered by a lack of information on the distribution and speciation of impurities within the cell, including the impact of internal reactions and gas crossover. Here we describe a novel operando method for detailed investigation of the impact of impurities on a single cell PEMFC, using a combination of isotopic labelling and measurement of gas composition at the anode exhaust via Gas Chromatography – Methaniser with Flame Ionisation Detector (GC-Methaniser-FID) and Selected Ion Flow Tube – Mass Spectrometry (SIFT-MS). We demonstrate the utility of this approach in the study of the impact of internal air bleed on carbon monoxide (CO) poisoning, enabling quantification of the surface coverage of CO on the anode catalyst as a function of cathode back-pressure. This technique shows great promise as a diagnostic tool for the investigation of the impact of a wide range of impurities at stack level (e.g. hydrocarbons, ammonia, halogenated compounds).

The effects of hydrogen dilution, carbon monoxide poisoning for a Pt–Ru anode in a proton exchange membrane fuel cell

The effects of carbon monoxide (CO) poisoning and hydrogen dilution are investigated for a series of simulated reformate gases. A simple mathematical model with an internal air bleed term is proposed to understand the interaction between CO poisoning, hydrogen dilution, and the internal air bleed (IAB). According to the steady-state modeling results, the presence of nitrogen can be considered as a diluting agent for a relatively dilute H2–N2 mixture. For a CO–H2–N2 mixture, hydrogen dilution and CO poisoning “magnify” each other in their combined effect on the surface coverage of hydrogen. The beneficial effect of IAB was found to be affected by the competitive adsorption between CO and hydrogen. The steady-state surface coverage of CO is influenced by the adsorption and desorption of CO, the interaction between CO and the electro-oxidation of hydrogen, the interaction between IAB/hydrogen dilution/CO, the interaction between IAB/CO. However, the steady-state surface coverage of CO is largely determined by its own adsorption and desorption properties. Analytical solutions are also derived from the proposed mode for the transient surface coverage of hydrogen and CO. The mathematical treatment has been proved extremely effective for the understanding of the steady-state and transient behavior of the CO–H2 and CO–H2–N2 mixtures.

CO Adsorption on Pd–Au Alloy Surface: Reversible Adsorption Site Switching Induced by High-Pressure CO

The interaction between carbon monoxide (CO) and a Pd70Au30(111) alloy surface was investigated under CO pressures with a wide range from ultrahigh vacuum (UHV) to sub-Torr at room temperature by a combination of near-ambient pressure (NAP) X-ray photoelectron spectroscopy and density functional theory calculations. The adsorption site and surface coverage of CO are reversibly controlled by the CO pressure. Under UHV conditions, the CO molecules occupy bridge and hollow sites on contiguous Pd clusters in the Au-rich surface layer. Exposure to sub-Torr CO gas induces site switching of the adsorbed CO on the contiguous Pd clusters from multiple-coordination (hollow and bridge) sites to single-coordination (top) sites, even though the latter sites are energetically less favorable. This behavior is explained by a pressure-induced entropic effect on gas-phase CO, which is in equilibrium with the adsorbed CO. This site switching highlights an important aspect of high-pressure-induced adsorption behavior.

Transient analysis of carbon monoxide transport phenomena and adsorption kinetics in HT-PEMFC during dynamic current extraction

This paper investigates the transport phenomena of carbon monoxide (CO) and adsorption kinetics, in a high-temperature proton exchange membrane fuel cell (HT-PEMFC) during step-wise current extraction (step-change in current extraction). Step-wise current extraction is a common process done to accommodate a sudden power surge during an operation. Since HT-PEMFCs are capable of handling high impurity of CO, hydrogen fuel that is contaminated with trace amount of CO is usually considered for commercial benefits. Thus, a transient three-dimensional isothermal anodic electro-kinetic numerical model is developed to determine the effect of operating parameters such as fuel cell temperature, CO inlet (initial) concentration, step-change of current density and dwell time on the transport phenomena of CO and adsorption kinetics. In addition, geometrical factors such as the catalyst layer (CL) and gas diffusion layer (GDL) porosity are also varied as well. The results show that the above-mentioned operating parameters can affect the maximum CO concentration at the CL, especially at the outlet of the channel. Specifically, a reduction of fuel cell temperature can significantly increase the CO concentration near the outlet, while increasing CO inlet (initial) concentration, step-change amplitude of current density and current density dwell time can cause an increase in CO concentration at the outlet, albeit to different extent. In addition, the increase in the porosity of CL and GDL, results in the reduction of the maximum CO concentration at the outlet, albeit to different extent. In addition, the CO and hydrogen surface coverage fractions are also affected by the variance of the above mentioned parameters.

Activation mechanism of carbon monoxide on α-Fe2O3 (0001) surface studied by using first principle calculations

We have employed first principle methods to investigate carbon monoxide (CO) adsorption on oxygen terminated α-Fe2O3 (0001) surface. We have found that the CO can be adsorbed and oxidized as long as the CO surface coverage reaches up to two thirds monolayer. Simultaneously, the bent CO2 are activated and dimerized. Furthermore, oxygen vacancy can dissociate water and produce carboxylic group by means of reactions with the activated CO2 dimer. Our calculations have reproduced experimental in situ observations of nanocrystalline α-Fe2O3. In this letter, we also propose a significant strategy to improve reactivity of α-Fe2O3 (0001) continuous film, to convert more efficiently CO into organic species.

Quantifying desorption and rearrangement rates of carbon monoxide on a PEM fuel cell electrode

A simple procedure to quantify the rates of carbon monoxide (CO) desorption from, and simultaneous rearrangement on, supported platinum fuel cell electrode (Pt on Vulcan XC-72R) is reported. The surface coverage of CO on Pt electrode in equilibrium with bulk CO was measured from the anodic peaks in the CO stripping voltammogram. The decline in these surface coverages due to desorption and rearrangement, once CO was replaced by N 2 in the gas phase was recorded and used in conjunction with a kinetic model to quantify the respective rates. Two distinct CO oxidation peaks observed in the voltammogram due to the oxidation of two distinct ad-species, namely weakly and strongly adsorbed CO ( C O ad I and C O ad II ), were baseline corrected and deconvoluted using a bimodal Gaussian distribution. Saturation surface coverage of C O ad I decreased with increasing temperature, while the opposite was true for C O ad II . Rearrangement from C O ad II to C O ad I was faster than the desorption rate of either CO species. The desorption rate of C O ad II was at least an order of magnitude lower than that of C O ad I molecules at all temperatures studied. The activation energies for desorption of C O ad I and C O ad II were estimated to be 24.08 and 27.99 kJ/mol, respectively. The activation energy for rearrangement from C O ad I to C O ad II was 35.23 kJ/mol and that from C O ad II to C O ad I was 27.55 kJ/mol.

Chemical oscillation in electrochemical oxidation of methanol on Pt surface

Based on dual path reaction mechanism, a nonlinear dynamics model reflecting the potential oscillation in electrooxidation of methanol on Pt surface was established. The model involves three variables, the electrode potential (e), the surface coverage of carbon monoxide (x), and adsorbed water (y). The chemical reactions and electrode potential were coupled together through the rate constant ki = exp(ai(e-ei)). The analysis to the established model discloses the following: there are different kinetics behaviors in different ranges of current densities. The chemical oscillation in methanol electrooxidation is assigned to two aspects, one from poison mediate CO of methanol electrooxidation, which is the induced factor of the chemical oscillation, and the other from the oxygen-containing species, such as H2Oa. The formation and disappearance of H2Oa deeply depend on the electrode potential, and directly cause the chemical oscillation. The established model makes clear that the potential oscillation in methanol electrooxidation is the result of the feedback of electrode potential e on the reactions involving poison mediates CO and oxygen-containing species H2Oa. The numerical analysis of the established model successfully explains why the potential oscillation in methanol galvanostatic oxidation on a Pt electrode only happens in a certain range of current densities but not at any current density.

Adsorption of Carbon Monoxide on Platinum in Hot Aqueous Acidic Solution from 423 to 533 K.

In order to evaluate adsorption of CO species onto precious metal electrodes during electrochemical oxidation of methanol under hot aqueous conditions, adsorption of carbon monoxide (CO) on a Pt electrode was studied in hot aqueous solution of 0.005 M H2SO4–0.2 M Na2SO4 by a fast potential sweep method at temperatures ranging from 423 to 533 K under the corresponding vapor pressure. An electrochemical half cell fabricated in a pressure vessel was employed to realize electrochemical measurement under this high temperature and pressure. Adsorption characteristics of CO were determined at CO partial pressure up to 0.39 MPa. Increasing temperature resulted in a decrease in the adsorption amount of CO on Pt electrodes. Under 0.1 MPa of the CO partial pressure, CO almost covers the electrode at 423 K, while a negligible amount is adsorbed at 533 K. At 473 K, CO surface coverage on a Pt electrode in aqueous solution increased with the CO partial pressure.

Periodic Density Functional Study of CO and OH Adsorption on Pt−Ru Alloy Surfaces: Implications for CO Tolerant Fuel Cell Catalysts

In this work, we present results of a periodic density-functional theory study of the adsorption of carbon monoxide (CO) and hydroxyl (OH) on platinum, ruthenium, and a series on platinum−ruthenium alloys. The surfaces are modeled as four-layer slabs. The binding energies and geometries of CO and OH are computed, as well as the vibrational properties of chemisorbed CO. We find that the mixing of Pt by Ru leads to a weaker bond of both CO and OH to the Pt sites, whereas mixing of Ru by Pt causes a stronger bond of CO and OH to the Ru sites. The binding energy trends for CO do not show a clear-cut relationship with its vibrational characteristics. The binding energy changes are electronic alloying effects that can be explained by the d band shift model of Hammer and Nørskov. From our calculations, we can conclude that for a good CO oxidation fuel cell catalyst, it is important to have both Pt sites (which bind CO weakly) and Ru sites (which bind OH strongly) on the surface. However, if a low surface coverage of CO is required, which may be case for the oxidation of H2 in the presence of a small amount of CO, Ru with a monolayer of Pt might be more advantageous, as this is the Pt−Ru surface that shows the weakest CO binding.

CO poisoning of [formula omitted]: Structural effects on the hydrogenolysis of methylcyclopropane

The hydrogenolysis of methylcyclopropane has been used as a probe reaction to investigate the structure sensitivity of the deactivation of a series of well-characterized PtAl2O3 catalysts by carbon monoxide.After a standardized reduction procedure, the catalysts were prepoisoned by pulse chemisorption of carbon monoxide at 298 K to varying levels of surface coverage, then the hydrogenolysis was carried out under differential conversion conditions at 273 K. Results were obtained in terms of reaction turnover frequency and selectivity as a function of percentage of metal exposed (Dh) and surface coverage of carbon monoxide. The general pattern of structure sensitivity for this reaction on PtAi2O3 was not altered substantially upon progressive poisoning, although the change in absolute magnitude of turnover frequency was larger for low-Dh catalysts than for high-Dh catalysts. Product selectivity between isobutane and n-butane was not affected. A comparison with prior results obtained for a similar series of PtSiO2 (Önal, I. and Butt, J. B., J. Chem. Soc. Faraday Trans, 178, 1887, 1982) is given. Comparison of prepoisoning from COAr pulses with that from COH2 pulses indicate differences for high-Dh catalysts. Thus there may be differences in apparent poisoning behavior as indicated by prepoisoning experiments compared to continuous introduction of poison with the reactants. Comparison of the mode of prepoisoning, pulse chemisorption vs thermal desorption from a monolayer, indicated generally similar results for the two methods. Some discrepancies appearing at low coverages of CO may be due to reordering of the adlayer in the thermal desorption treatment.