Kinetics Of CO Oxidation Research Articles

Langmuir–Hinshelwood Mechanism Including Lateral Interactions and Species Diffusion for CO Electro-Oxidation on Metallic Surfaces

The electrochemical oxidation of CO on metallic surfaces following a lattice-gas model, including effective lateral interactions between one of the adsorbates and species diffusion, is studied. The reaction occurs through a Langmuir–Hinshelwood (LH) mechanism in which adsorbed CO reacts with adsorbed hydroxyl species. The mean field approximation and dynamic Monte Carlo simulations have been compared. The effect of the molecular distribution and surface mobility of reacting species on the potential dependence of the CO oxidation rate is analyzed. The inclusion of lateral interactions into the reaction mechanism reconciles different experimental observations, such as island formation and fast CO diffusion. Results highlight the importance of effective interactions in the reaction kinetics and suggest that they should be taken into account when interpreting experimental data. Simulations are useful for an improved qualitative understanding of the kinetics of CO oxidation and other electrochemical LH reactions.

Enhancing the low temperature oxidation performance over a Pt and a Pt–Pd diesel oxidation catalyst

The influence of hydrogen over platinum and combined platinum–palladium diesel oxidation catalysts were investigated on the oxidation kinetics of CO, HC and NO. Although H2 has been reported to have a positive effect on CO and HC oxidation as well as NO2 formation over platinum catalysts, there is still uncertainty whether this is due to the temperature rise caused by H2 oxidation or the result of a change in the reaction kinetics of CO, HC and NO oxidation by the production of intermediate species. The results have showed smaller H2 concentrations are more effective in improving the catalyst light-off temperature as well as promoting NO oxidation over both platinum and platinum–palladium catalysts. It is suggested that these benefits are a result of not only the exothermic reactions which in turn increase the local catalyst temperature but also H2 increasing the rate of reactions and the species accessibility to the catalyst active sites thus further CO, HC and NO oxidation can occur at lower catalyst temperatures.

Developing global reaction rate model for CO oxidation over Au catalysts from past data in literature using artificial neural networks

In this work, the literature for CO oxidation kinetics over Au based catalysts was analyzed using artificial neural networks to test the possibility of developing global reaction rate models representing the entire literature. A database was constructed using the data obtained from nineteen papers published between the years 1997 and 2011; then, the reaction rate was modeled as a function of catalyst preparation and operational variables by using neural networks. Next, global reaction rate equations in the form of power law were developed for each support type by the help of the neural network model, and the order of reaction with respect to each reactant and the parameters of Arrhenius relation were estimated. These power law models were successfully validated by using the information reported in the literature; hence, it was concluded that they can be used for the initial estimation of the reaction rates in the absence of more specific rate equations.

Tuning the electrochemistry of homoleptic cobalt 4,4′-disubstituted-2,2′-bipyridine redox mediators

The electrochemistry of a series of Co(II) tris(4,4′-disubstituted-2,2′-bipyridine) hexafluorophosphate complexes was investigated in acetonitrile, and compared to the parent, un-substituted 2,2′-bipyridine complex. The introduction of an electron-withdrawing halogen substituent increased the formal redox potential of the couple by 190–270mV, in the order of F>Cl>Br>I. Substitution with electron-donating tert-butyl and methoxy groups acted as electron donating groups, and decreased the formal potential by 150–190mV. Substitution in the 4,4′ positions, irrespective of electron withdrawing or donating character, also imparted an asymmetry in the anodic peak in the cyclic voltammograms, where the oxidation process became more sluggish than the corresponding reduction. Kinetics of Co(II) oxidation was evaluated by Koutecký–Levich analysis at a rotating Pt disc, revealing superior oxidation kinetics of the un-substituted 2,2′-bipyridine complex by up to an order of magnitude or larger. The anodic transfer coefficients were also extracted from this analysis, revealing ideal α=0.50 for the parent complex, to 0.70≤α≤0.88 for complexes with substituted ligands.

The Role of Defects in the Local Reaction Kinetics of CO Oxidation on Low-Index Pd Surfaces.

The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10–5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.

Rh1/γ‐Al2O3 Single‐Atom Catalysis of O2 Activation and CO Oxidation: Mechanism, Effects of Hydration, Oxidation State, and Cluster Size

AbstractThe single‐atom catalysis of O2 activation and CO oxidation with Rh1 supported on γ‐Al2O3 is investigated here through ab initio molecular dynamics techniques. We scrutinize the molecular details of the mechanism for the full catalytic cycle that involves the oxidation of two CO molecules in succession. The effect of the surface hydration and oxidation state of Rh on the kinetics of O2 activation and CO oxidation is presented. We also report here the catalytic activity of experimentally intercepted RhI(CO)2 on γ‐Al2O3. Furthermore, we delineate the importance of single‐atom catalysis by comparing the performance of the Rh6/Al2O3 catalyst. A molecular level understanding of the differential reactivity on hydration, on oxidation, and of a larger Rh cluster size is reported.

Structural analysis of carbon-supported Ru-decorated Pt nanoparticles synthesized using forced deposition and catalytic performance toward CO, methanol, and ethanol electro-oxidation

Carbon-supported Ru-modified Pt nanoparticles (Ru(Pt)/C), prepared using forced deposition under H2 stream, were tested as anodic fuel cell electrocatalysts. Ru nanoislands, mainly composed of RuO, RuO2, and hydrous RuO2 (RuOxHy), were deposited on the Pt nanoparticles. The coverage of Pt by Ru species (θ) depended on the Ru salt concentration and the treatment duration, the deposition process following an adsorption-controlled Elovich-type kinetics. The CO oxidation kinetics was optimal for θ≈0.50, whereas for both, methanol and ethanol, the highest activity was found for θ≈0.15–0.20. The observed promotional effect was attributed to the reaction of hydroxylated Ru species with strongly adsorbed CO and related intermediates. The forced deposition technique appears to be a suitable simple method for the preparation of Ru(Pt) nanoparticles with improved fuel oxidation performance. The Tafel analysis allowed correlating the reaction pathway and the limiting step with the coverage by Ru.

Study of high temperature oxidation kinetics of Co–Al coating on superalloy

Nanostructured Co–Al coating on Superni-718 substrate was deposited by dc/rf magnetron sputtering. The high temperature oxidation kinetics of Co–Al coating on superalloy substrate was studied after 100 h of cyclic oxidation in air environment at 700–900°C. FE-SEM/EDS, AFM and XRD were used to characterise the microstructure, surface morphology and phase constituents of as deposited and oxidised coatings. The results indicate that the Co–Al coated superalloy substrate has higher oxidation resistance at 900°C. The presence of β-CoAl intermetallic phase in the Co–Al coating, the formation of Al2O3 and spinel phases such as CoCr2O4 and CoAl2O4 in the oxide scale are considered to be the reasons for the higher oxidation resistance shown by the Co–Al coated superalloy substrate after 100 h of cyclic oxidation in air at 900°C. Further, the oxide scale formed on the coated sample exhibits good spallation resistance during cyclic oxidation test at 900°C.

Electrocatalytic oxidation of methanol and carbon monoxide at platinum in protic ionic liquids

Oxidation of H2O, CO and CH3OH was investigated at Pt as a function of temperature in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Trace H2O oxidation in the ionic liquid results in coverage of the Pt with adsorbed oxides. Increasing the temperature significantly reduces the potential at which this reaction occurs. CH3OH and CO oxidation kinetics increased significantly with increasing temperature and oxidation of each species coincided with coverage of the Pt surface by the oxide. These observations indicate that surface oxides are required for complete oxidation of CH3OH to CO2 in the protic ionic liquid, in a similar way to that observed in conventional aqueous electrolytes. While the overpotential for CH3OH oxidation was drastically higher than that observed in purely aqueous electrolytes, it decreased with increasing water content of the ionic liquid. The results described here have implications for the development of protic ionic liquid electrolyte fuel cells and these implications are discussed.

A computationally efficient steady-state electrode-level and 1D + 1D cell-level fuel cell model

Computational efficiency is highly important for upscaling detailed electrode-level and cell-level models to the system level required for the design and control of fuel cells. We present a computationally efficient 1D+1D fuel cell model based on a combination of analytical and numerical approaches. On the electrode level, we develop approximate analytical solutions for the 1D current/potential distribution via a hybrid algorithm of power-law approach and perturbation method. Compared to the conventional perturbation method, this work keeps the intrinsic nonlinearity of electrochemical kinetics, while providing clearer physical meaning than some purely mathematical methods like the Adomian decomposition method. By integrating the resulting overpotential profile into mass transfer models, concentration overpotentials are obtained and the thermodynamic framework is then used for analyzing the H2/CO electrochemical co-oxidation kinetics. A novel expression is also presented to interconvert volume- and area-specific exchange current densities. On the cell level, a linear relationship between local current density and solid temperature is further developed for efficient 1D+1D thermal along-the-channel numerical simulations without requiring computational iterations. Both the electrode-level and cell-level macroscopic fuel cell models are validated against full numerical solutions available in the open literatures over a wide range of operating conditions. With the hybrid analytical/numerical approximation in two dimensions, the computational framework is predicted to be sufficiently efficient for real-time simulations.

Catalytic oxidation of CO on platinum

Using concepts recently developed in thermal decompositions of solids and reduction of bulk oxides by gases and (re)analysis of experimental literature data, a novel mechanism for the catalytic oxidation of CO by PtO2 is proposed. Instead of the conventional Mars–van Krevelen scheme, the reactions proposed are: PtO2(s) + 2CO ⟷ Pt(g) + 2CO2 and Pt(g) + O2 ⟷ PtO2(g) → PtO2(s). The first reaction determines the kinetics of CO oxidation and the second determines the kinetics of restoration of the PtO2 layer. Thermochemical consideration of the kinetic features of this model, based on Langmuir’s quasi-equilibrium equations for evaporation of simple substances, allowed calculation of the reaction enthalpy and the absolute rate of CO oxidation. These results are in good agreement with experimental data. The proposed mechanism explains the origin of the surface-retexturing effect, the limited loss of Pt metal from the catalyst, the mechanism of PtO2 regeneration by oxygen, the strong effect of CO2 in reducing the CO oxidation rate and the three-fold variation of the Arrhenius E parameter with temperature.

On the promoting effect of Au on CO oxidation kinetics of Au–Pt bimetallic nanoparticles supported on SiO2: An electronic effect?

Bimetallic AuPt nanoparticles prepared by radiolysis with Au/Pt atomic ratios of 1.2 and 1.8 were deposited onto silica and calcined to remove the stabilizing polymers. During this sequence, they lose their core–shell structure and restructure as alloys. FTIR and XPS provide evidence for further AuPt NP restructuring under CO exposure and CO oxidation with Pt surface enrichment. A promoting effect of Au is found on the CO oxidation kinetics in the AuPt catalyst with the lower Au/Pt ratio. This effect is attributed to an electron enrichment of the Pt surface atoms due to charge transfer from Au to Pt, as indicated by XPS and CO-FTIR.

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.

Strontium transport and conductivity of Mn 1.5Co 1.5O 4 coated Haynes 230 and Crofer 22 APU under simulated solid oxide fuel cell condition

Interaction of current collectors with coated interconnect alloys was examined using dual-coating specimens. Oxidation kinetics of (La,Sr)(Co,Fe)O 3 (LSCF)/Mn 1.5Co 1.5O 4 (MCO) and (La,Sr)(Mn)O 3 (LSM)/MCO coated Haynes 230 (H230) was determined at 750 °C in air via a gravimetric method. The LSCF/MCO dual coating samples exhibited a rate constant 2 times larger than that of the LSM/MCO coated H230. SrCrO 4 that formed at the LSCF/MCO coated H230 interface contributed to the increased rate constant. No SrCrO 4 was detected at the interface of the LSM/MCO coated H230. Chemical stability of LSCF in air at 750 °C was confirmed by TGA. Area specific resistance (ASR) of MCO coated Haynes 230 (H230) and Crofer 22 APU with LSCF and LSM (coated H230 only) current collectors was measured at 800 °C for 1100 h. No gross difference in ASR evolution was observed between the LSCF/MCO-H230 and LSM/MCO-H230 assemblies. Activation energy of conduction of the LSCF/MCO-H230 and LSCF/MCO-Crofer 22 samples evolved from 0.63 eV to 0.80 eV and from 0.53 eV to 0.63 eV respectively over a period of ca. 1100 h, suggesting somewhat different interface chemistry evolution. Sr transport was observed in the LSCF/MCO-H230 samples subjected to both exposure test and ASR measurement, whereas it was not detected on the LSCF/MCO-Crofer 22 sample. Mechanism of the Sr transport is discussed on a qualitative basis to account for the observations.

Kinetic Modeling Study of the Oxidation of Carbon Monoxide–Hydrogen Mixtures over Pt/Al2O3and Rh/Al2O3Catalysts

Detailed kinetic modeling was used to interpret the kinetics of the global CO + O2 reaction in the presence of hydrogen on Pt/Al2O3, Rh/Al2O3, and Pt/Rh/Al2O3 catalysts. The rate parameters were compiled from the literature and optimized to improve the predictivity against multiple experimental data sets. The intrinsic kinetics of CO oxidation in the presence of hydrogen was well described for Pt/Al2O3 and Rh/Al2O3 catalysts. On the basis of microkinetic studies, a global Langmuir–Hinshelwood rate expression for platinum and rhodium was derived. The resulting global rate was successfully implemented in the AMESim modeling platform. Both the detailed and global models predict correctly the CO conversion profiles and the enhancement effect of H2 to the CO light-off temperatures in the CO/O2/H2 system. This promoting effect is attributed to the surface reaction between CO(s) and OH(s) which represents an alternate way for consuming CO(s).

Analysis of reaction mechanisms and kinetics of preferential CO oxidation over Au/γ-Al2O3

In this work, the mechanism and kinetics of the preferential CO oxidation over 0.5 wt. % Au/Al2O3 catalyst was investigated. For CO oxidation, the reaction was found to follow a two-step Eley–Rideal-type mechanism, in which the formation and decomposition of the reaction intermediate took place at the gold-support interface. For the PROX case, on the other hand, Langmuir–Hinshelwood-type mechanisms seem to be also significant.

Erratum to: The Kinetics of CO Oxidation by Adsorbed Oxygen on Well-Defined Gold Particles on TiO2(110)

On page 149, the text of this paper reads: ‘‘Using simple Redhead analysis [28] and assuming first order desorption kinetics and a pre-exponential factor for desorption of 5.5 9 10 s from a closely related O/Au system [22], we estimate desorption activation energies ranging from 124.7 kJ/mol (for the *1-atom thick islands) to 90.4 kJ/mol (for the 6-atom thick islands). Since the activation energy for desorption generally follows the same trend as the adsorption energy (and they are equal for nonactivated adsorption), we conclude that the *34.3 kJ/mol higher activation energy on the thinnest particles corresponds to a larger adsorption energy (i.e., considerably more stable Oa) on the thinnest particles of Au.’’ There was a calculation error here, and these three energies must be changed so that this text instead reads: ‘‘Using simple Redhead analysis [28] and assuming first order desorption kinetics and a pre-exponential factor for desorption of 5.5 9 10 s from a closely related O/Au system [22], we estimate desorption activation energies ranging from 190 kJ/mol (for the *1-atom thick islands) to 139 kJ/mol (for the 6-atom thick islands). Since the activation energy for desorption generally follows the same trend as the adsorption energy (and they are equal for nonactivated adsorption), we conclude that the *51 kJ/mol higher activation energy on the thinnest particles corresponds to a larger adsorption energy (i.e., considerably more stable Oa) on the thinnest particles of Au.’’ Correspondingly, the value of ‘‘34.3 kJ/mol’’ appearing in three other places (on Figure 9, in the right-hand column of page 149 and the left-hand column of page 150) all must be corrected also to be ‘‘51 kJ/mol’’. Similarly, the value ‘‘[17.2 kJ/mol’’ appearing in two places (on Figure 10 and in the left-hand column of page 150) must be corrected in both places to be ‘‘[25 kJ/mol’’. The authors thank Dr. Beatriz Roldan Cuenya for pointing out this error.

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513K. A change in the front propagation mechanism is proposed to explain the observed effects.

Kinetic study of CO oxidation on step decorated Pt(1 1 1) vicinal single crystal electrodes

In this work, surface modification at atomic level was applied to study the reactivity of step sites on platinum single crystal surfaces. Stepped platinum single crystal electrodes with (1 1 1) terraces separated by monoatomic step sites with different symmetry were decorated with irreversibly adsorbed adatoms, without blocking the terrace sites, and characterized in 0.1 M HClO 4 solution. The kinetics of CO oxidation on the different platinum single crystal planes as well as on the step decorated surfaces has been studied using chronoamperometry. The apparent rate constants, which were determined by fitting the experimental data to a mean-field model, decrease after the steps of platinum single crystal electrodes have been blocked by the adatoms. This behavior indicates that steps are active sites for CO oxidation. Tafel slopes measured from the potential dependence of the apparent rate constants of CO oxidation were similar in all cases. This result demonstrates that the electrochemical oxidation of the CO adlayer on all the surfaces follows the same Langmuir–Hinshelwood model, irrespectively of step modification.

Electrical properties and reactivity under air–CO flows of composite systems based on ceria coated carbon nanotubes

Nanocomposite systems of ceria nanoparticles and Double Walled Carbon Nanotubes (DWNTs) coated with nano-ceria (n-CeO 2) were elaborated using a classical sol gel method. Three samples noted as [n-CeO 2 + xDWNTs] with variable weight fractions x = 0, 5 and 15 wt.% of DWNTs were obtained. The samples were characterized by X-ray diffraction and electron microscopy. The electrical conductivity of [n-CeO 2 + xDWNTs] compacted pellets systems was determined from electrical impedance spectrometry, under air, between 120 and 400 °C. In this temperature range, all samples were semi-conducting with a weak variation of activation energy. However, the conductivity strongly increased with the weight fraction of ceria coated carbon nanotubes. Finally, the solid–gas interactions between air–CO flows and these systems were studied as a function of time and temperature, by means of Fourier transform infrared (FTIR) spectroscopy. The oxidation kinetics of CO into CO 2 was analyzed from the evolutions of FTIR absorption band intensities. As the carbon nanotube fraction x increased, the conversion efficiency was strongly improved.