Step Of CO Oxidation Research Articles

Catalytically Active and Spectator Ce3+ in Ceria‐Supported Metal Catalysts

AbstractIdentification of active species and the rate‐determining reaction steps are crucial for optimizing the performance of oxygen‐storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce3+ species in a ceria‐supported platinum catalyst during CO oxidation are short‐lived and therefore cannot be observed under steady‐state conditions. Using time‐resolved resonant X‐ray emission spectroscopy, we quantitatively correlated the initial rate of Ce3+ formation under transient conditions to the overall rate of CO oxidation under steady‐state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce3+ was present as spectators. This approach can be applied to various catalytic processes involving oxygen‐storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.

Surface electrochemistry of CO2 reduction and CO oxidation on Sm-doped CeO(2-x): coupling between Ce(3+) and carbonate adsorbates.

The efficient electro-reduction of CO2 to chemical fuels and the electro-oxidation of hydrocarbons for generating electricity are critical toward a carbon-neutral energy cycle. The simplest reactions involving carbon species in solid-oxide fuel cells and electrolyzer cells are CO oxidation and CO2 reduction, respectively. In catalyzing these reactions, doped ceria exhibits a mixed valence of Ce(3+) and Ce(4+), and has been employed as a highly active and coking-resistant electrode. Here we report an operando investigation of the surface reaction mechanism on a ceria-based electrochemical cell using ambient pressure X-ray photoelectron spectroscopy. We show that the reaction proceeds via a stable carbonate intermediate, the coverage of which is coupled to the surface Ce(3+) concentration. Under CO oxidation polarization, both the carbonate and surface Ce(3+) concentration decrease with overpotential. Under CO2 reduction polarization, on the other hand, the carbonate coverage saturates whereas the surface Ce(3+) concentration increases with overpotential. The evolution of these reaction intermediates was analyzed using a simplified two-electron reaction scheme. We propose that the strong adsorbate-adsorbate interaction explains the coverage-dependent reaction mechanism. These new insights into the surface electrochemistry of ceria shed light on the optimization strategies for better fuel cell electrocatalysts.

Kinetic study of CO oxidation over CuO/MO2 (M = Si, Ti and Ce) catalysts

A series of supported CuO/MO2 catalysts (M=Si, Ti and Ce) were prepared using a chemisorption-hydrolysis method and tested for CO oxidation. Kinetic study was carried out to investigate reaction pathways on these catalysts. The power-rate law reaction expressions over CuO/SiO2, CuO/TiO2 and CuO/CeO2 were r=1.78×10−7Pco0.85Po20.22, r=1.62×10−7Pco0.65Po20.19 and r=1.5×10−7Pco0.7Po20, respectively. Activation energies were calculated to be 74.8, 53.4 and 46.3kJmol−1 for the CuO/SiO2, CuO/TiO2 and CuO/CeO2 catalysts respectively. Elementary steps of CO oxidation were also proposed. For the CuO/SiO2 catalysts, a Langmuir–Hinshelwood (L–H) model involving chemisorption of CO and O2 on surface Cu atoms was proposed; for the CuO/TiO2 catalysts, a L–H model involving chemisorption of CO on Cu atoms and chemisorption of O2 on TiO2 was proposed, which resulted in a interfacial reaction between the two species. For the CuO/CeO2 catalysts, a Mars van–Krevelen model involving chemisorbed CO on Cu atoms reacting with lattice oxygen in CeO2 was proposed. Effect of CuO particle size was also evaluated. It was found that the active site on the periphery of the CuO–TiO2 and CuO–CeO2 interface was more active on large CuO crystallite than that on small one. The enhanced activity could be interpreted by the higher concentration of chemisorbed CO on the active site on the larger CuO crystallite.

Single Layer of Polymeric Cobalt Phthalocyanine: Promising Low‐Cost and High‐Activity Nanocatalysts for CO Oxidation

The catalytic behavior of transition metals (Sc to Zn) combined in polymeric phthalocyanine (Pc) is investigated systematically by using first-principles calculations. The results indicate that CoPc exhibits the highest catalytic activity for CO oxidation at room temperature with low energy barriers. By exploring the two well-established mechanisms for CO oxidation with O2 , namely, the Langmuir-Hinshelwood (LH) and the Eley-Rideal (ER) mechanisms, it is found that the first step of CO oxidation catalyzed by CoPc is the LH mechanism (CO + O2 → CO2 + O) with energy barrier as low as 0.65 eV. The second step proceeds via both ER and LH mechanisms (CO + O → CO2 ) with small energy barriers of 0.10 and 0.12 eV, respectively. The electronic resonance among Co-3d, CO-2π*, and O2 -2π* orbitals is responsible for the high activity of CoPc. These results have significant implications for a novel avenue to fabricate organometallic sheet nanocatalysts for CO oxidation with low cost and high activity.

Structural Selectivity of CO Oxidation on Fe/N/C Catalysts

Fe/N/C electrocatalysts are attractive for electrocatalytical applications in fuel cells due to their low cost and high electrocatalytic activity. Because of its complex nature, the mechanism of CO oxidation on Fe/N/C electrocatalysts remains largely unknown. Aiming to provide insight into the mechanism of CO oxidation, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Fe/N/C active sites, including Fe-N4 and Fe-N3 porphyrin-like carbon nanotube (T-FeN4 and T-FeN3), Fe-N4 porphyrin-like graphene (G-FeN4), and Fe-N2 nanoribbon (R-FeN2). It is found that CO adsorption and oxidation are very sensitive to the active site structures. CO adsorption is energetically more favorable than O2, and CO oxidation to CO2 hardly occurs on T-FeN4 and G-FeN4. In comparison, O2 prefers to adsorb and CO can easily be oxidized on T-FeN3 and R-FeN2, suggesting the CO tolerant property of T-FeN3 and R-FeN2.

Unraveling the Mechanisms of O2 Activation by Size-Selected Gold Clusters: Transition from Superoxo to Peroxo Chemisorption

The activation of dioxygen is a key step in CO oxidation catalyzed by gold nanoparticles. It is known that small gold cluster anions with even-numbered atoms can molecularly chemisorb O(2) via one-electron transfer from Au(n)(-) to O(2), whereas clusters with odd-numbered atoms are inert toward O(2). Here we report spectroscopic evidence of two modes of O(2) activation by the small even-sized Au(n)(-) clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O(2)Au(8)(-) revealed two distinct isomers, which can be converted from one to the other depending on the reaction time. Ab initio calculations show that there are two close-lying molecular O(2)-chemisorbed isomers for O(2)Au(8)(-): the lower energy isomer involves a peroxo-type binding of O(2) onto Au(8)(-), while the superoxo chemisorption is a slightly higher energy isomer. The computed detachment transitions of the superoxo and peroxo species are in good agreement with the experimental observation. The current work shows that there is a superoxo to peroxo chemisorption transition of O(2) on gold clusters at Au(8)(-): O(2)Au(n)(-) (n = 2, 4, 6) involves superoxo binding and n = 10, 12, 14, 18 involves peroxo binding, whereas the superoxo binding re-emerges at n = 20 due to the high symmetry tetrahedral structure of Au(20), which has a very low electron affinity. Hence, the two-dimensional (2D) Au(8)(-) is the smallest anionic gold nanoparticle that prefers peroxo binding with O(2). At Au(12)(-), although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O(2).

CO oxidation catalyzed by a single gold atom: benchmark calculations and the performance of DFT methods

Quantum chemical calculations were carried out on CO oxidation catalyzed by a single gold atom. To investigate the performance of density functional theory (DFT) methods, 42 DFT functionals have been evaluated and compared with high-level wavefunction based methods. It was found that in order to obtain accurate results the functionals used must treat long range interaction well. The double-hybrid mPW2PLYP and B2PLYP functionals are the two functionals with best overall performance. CAM-B3LYP, a long range corrected hybrid GGA functional, also performs well. On the other hand, the popular B3LYP, PW91, and PBE functionals do not show good performance and the performance of the latter two are even at the bottom of the 42 functionals. Our accurate results calculated at the CCSD(T)/aug-cc-pVTZ//mPW2PLYP/aug-cc-pVTZ level of theory indicate that Au atom is a good catalysis for CO oxidation. The reaction follows the following mechanism where CO and O(2) adsorb on Au atom forming an Au(OCOO) intermediate and subsequently O(2) transfer one oxygen atom to CO to form CO(2) and AuO. Then AuO reacts with CO to form another CO(2) to complete the catalytic cycle. The overall energy barrier at 0 K for the first CO oxidation step (Au + CO + O(2)→ AuO + CO(2)) is just 4.8 kcal mol(-1), and that for the second CO oxidation step (AuO + CO → Au + CO(2)) is just 1.6 kcal mol(-1).

Analytical approach for estimating kinetics of elementary steps on transition metals: Tunneling, trends and Brönsted–Evans–Polanyi relation

An overall formulation of an adiabatic approach, which combines adsorption characteristics of reactants and products with an analytical method of searching for the saddle point (SP) on semi-empirical adiabatic potential energy surface (APES), is presented for studying surface reactions kinetics. The following cases are analyzed: (i and ii) breaking or formation of a chemical bond, (iii) diffusion hopping of a molecule on a metal surface, and (iv) subsurface penetration of a metal surface. Numerical results for the activation energies and rate constants of elementary steps of CO oxidation, oxygen dissociative adsorption and water dissociation on transition metals (Pt, Pd, etc.) at the zero-coverage limit are presented. Specifically, the reaction energy dependencies of the kinetic characteristics like activation energy are presented as Brönsted–Evans–Polanyi relation. Moreover, the effect of tunneling along reaction coordinate is shown to be significant for several steps. The calculated diffusitivities of CO on (111) surfaces of several metals are correlated with the corresponding adsorption energies. The mechanism of subsurface oxygen penetration in a cluster resembling a (111) palladium surface is investigated. Approximate dependence of the activation energy of the oxygen subsurface diffusion on surface coverage is suggested.

Metal-Embedded Graphene: A Possible Catalyst with High Activity

The catalytic activity of Au-embedded graphene is investigated by the first-principle method using the CO oxidation as a benchmark probe. The first step of CO oxidation catalyzed by the Au-embedded graphene is most likely to proceed with the Langmuir−Hinshelwood reaction (CO + O2 → OOCO → CO2 +O), and the energy barrier is as low as 0.31 eV. The second step of the oxidation would be the Eley−Rideal reaction (CO + O → CO2) with a much smaller energy barrier (0.18 eV). The partially filled d states of Au are localized around the Fermi level due to the interactions between Au and the neighboring carbon atoms. The high activity of Au-embedded graphene may be attributed to the electronic resonance among electronic states of CO, O2, and the Au atom, particularly, among the d states of the Au atom and the antibonding 2π* states of CO and O2. This opens a new avenue to fabricate low cost and high activity carbon-based catalyst.

On the Structure Sensitivity of CO Oxidation on Alumina Supported Pd--Pt Bimetallic Catalysts

CO oxidation reaction was studied over monometallic and bimetallic palladium--platinum catalysts at 25:75, 50:50, and 75:25 Pd:Pt atomic fractions co-impregnated over a \gamma -Al_2O_3 support. The size of the metal particles, measured by modified hydrogen chemisorption, increased as the Pd fractions in the catalyst increased. The surface compositions and site metal distributions of the catalysts determined from Monte Carlo simulations indicated that Pd atoms segregated to the surface: at low Pd levels, Pd occupied preferentially 6 and 7 coordinated defect-like sites. As the Pd fraction increased, first 8 coordinated (100) and then 9 coordinated (111) planes were populated. At low temperatures (below 423 K), CO oxidation reaction over bi-metallic catalysts behaved like the monometallic Pd catalyst suggesting that Pd atoms segregated to the surface. The results of Monte Carlo studies and reaction tests together were interpreted as the governing steps in CO oxidation primarily taking place at the defect-like sites.

Kinetic modeling of the oxidation of CO on Fe 2O 3 catalyst in excess of O 2

This work addresses the oxidation of CO under oxygen-rich conditions using a Fe 2O 3 model catalyst. Based on in situ DRIFTS studies, isotopic labeling, and kinetic examinations performed in a gradient-free loop reactor an Eley–Rideal type mechanism is postulated. This mechanism includes the dissociative adsorption of O 2 on active Fe sites, followed by reaction of surface oxygen with gaseous CO, producing CO 2. Furthermore, a mean field model is constructed for numeric modeling and simulation of the CO oxidation, as well as calculation of the Fe 2O 3 surface coverage. The kinetic model represents a network of six elementary reactions using Arrhenius-based rate expressions. The comparison between measured and calculated data shows that the model describes the experiments well. Kinetic parameters for the elementary reactions are obtained from the literature or by fitting calculations. To reduce the number of free parameters, the patterns of O 2 TPD and CO 2 TPD are modeled numerically. To validate the model, the kinetic parameters are used to simulate catalytic data, which agree fairly well with the corresponding experimental results. The reaction of surface oxygen species with gas-phase CO is considered to be the rate-determining step in CO oxidation on an Fe 2O 3 catalyst. In addition, the thermodynamic consistency of the kinetic parameters is proven.

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe2O3–CeO2 composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe2O3–CeO2 composites with an Fe molar percentage below 0.3 form solid solutions with the CeO2 cubic fluorite structure, in which the doped Fe3+ initially substitutes Ce4+ in fluorite cubic CeO2, but then mostly locate in the interstitial sites after a critical concentration of doped Fe3+. With an Fe molar percentage between 0.3 and 0.95, the Fe2O3–CeO2 composites are mixed oxides of the cubic fluorite CeO2 solid solution and the hematite Fe2O3. XPS results indicate that CeO2 is enriched in the surface region of Fe2O3–CeO2 composites. The Fe2O3–CeO2 composites have much higher catalytic activities in CO oxidation than the individual pure CeO2 and Fe2O3, and the Fe0.1Ce0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe2O3–CeO2 composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO2-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.

CO oxidation on rough Au thin films grown on Si wafer

Abstract The CO oxidation reactivity of rough Au thin films containing ∼5% W was investigated. At room temperature, no CO oxidation could be detected under our experimental conditions, whereas conversion of CO to CO 2 was observed at 160 °C. At higher temperatures, the initial reactivity increased. However, with increasing reaction time, it was evident that deactivation of the catalytically active sites was more facile at higher temperatures. Temperature-programmed desorption (TPD) suggested that the formation of strongly bound carbonate species could be responsible for the deactivation process. Based on the TPD data, we propose that decomposition of carbonate species on the surface is the rate-determining step of CO oxidation.

CO Adsorption and Oxidation on Ceria Surfaces from DFT+U Calculations

The limiting steps of CO oxidation catalyzed by ceria via the Mars−van Krevelen reaction mechanism are identified and investigated by means of density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U). We address the adsorption of CO on the (111) and (110) surfaces, and its oxidation via participation of lattice oxygen leading to vacancy formation and CO2 desorption. CO physisorbs on the (111) ceria surface (Ebind 2 eV) yielding carbonate formation and surface reduction. The DFT+U energy of CO adsorption and oxidation is studied as a function of the parameter U. This analysis demonstrates that the values of U presently used in the literature (U > 4 eV) tend to overestimate the binding energy of CO to ceria surfaces. In contrast, the energy for the complete CO oxidation reaction is effectively independent of U and in good agreement with the available experimental data. The discuss...

Effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation under dynamic oxygen storage and release measurements over Pd/CeO2 catalysts

A kinetic mathematical model has been applied to investigate for the first time the effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation during Oxygen Storage Capacity (OSC) measurements under dynamic conditions over Pd/CeO2 catalysts in the 500–700 °C range. The dependence of the intrinsic rate constant k1 of the CO oxidation reaction on PdO, and that of k 2 app of the oxygen back-spillover from ceria to Pd/PdO on the palladium particle size was estimated by performing curve-fitting of the experimental CO and CO2 pulse transient responses obtained. Activation energies of 8.0, 9.5 and 21.1 kJ/mol were calculated for the Eley–Rideal step of CO oxidation for the 1.3, 1.8 and 16.4 nm Pd particles, respectively, supported on CeO2. The transient rates of CO oxidation and oxygen back-spillover were found to decrease with increasing Pd particle size.

A first principles analysis of the electro-oxidation of CO over Pt(1 1 1)

First principle density functional theoretical calculations carried out within a constant potential half-cell formalism were used to model the electro-oxidation of CO over Pt(1 1 1). The method involves tuning the potential by the addition or removal of electrons from the system. The free energy for different adsorbed species within the double-layer is analyzed over a range of different potentials to establish the lowest energy states and the reaction energies that connect these states. The potentials are calculated based on a novel double-reference approach [J.S. Filhol, M. Neurock, Angew. Chem. Int. Ed. 45 (2006) 402] discussed earlier. The potential-dependent reaction energies are reported for the elementary steps of water activation in the presence of co-adsorbed CO and CO oxidation over the model Pt(1 1 1) surface. The potential-dependent activation barriers are computed for the key elementary steps in CO oxidation to develop a detailed reaction energy profile as a function of electrode potential. The results suggest that the coupling of co-adsorbed CO and OH controls the rate. Water activation, however, is necessary to supply a critical coverage of the surface OH oxidant.

Transient studies of the mechanisms of CO oxidation over Au/TiO 2 using time-resolved FTIR spectroscopy and product analysis

Transient studies on the oxidation of CO over Au/TiO 2 have been performed at 243–363 K using a time-resolved fast-scanning Fourier transform infrared (FTIR) spectrometer coupled with a quadrupole mass spectrometer. Different CO–O 2 interactions were performed by stepping CO into a steady-state He flow, O 2 into a steady-state CO flow, CO into a steady-state O 2 flow, and CO/O 2 into an He flow. The CO oxidation reaction occurs at all temperature ranges investigated whether CO is preadsorbed or O 2 is preadsorbed, indicating that there is no competition between CO and O 2 for adsorption at a temperature range of 243–363 K (noncompetitive adsorption). It suggests that CO and O 2 adsorption occurs on two different types of sites. When there is no O 2 present in step feed (i.e., step CO into He), CO immediately builds up on the surface (rapid adsorption of CO) and desorbs slowly after cutting off CO flow. However, when O 2 is present in the step feed (i.e., step CO/O 2 into He), adsorbed CO reacts with adsorbed oxygen immediately (fast surface reaction), whereas CO 2 product desorption monitored by mass spectroscopy appears to be very slow. Slow desorption of CO 2 product is claimed to be the rate-limiting step of CO oxidation over Au/TiO 2 at a temperature range of 243–273 K.

Impact of Pt dispersion on the elementary steps of CO oxidation by O 2 over Pt/Al 2O 3 catalysts

The impact of Pt dispersion (denoted by D) of Pt/Al 2O 3 catalysts on the turnover frequency (TOF) of the CO/O 2 reaction and on the elementary steps involved in the catalytic reaction is studied using transient experiments with either a mass or a FTIR spectrometer as a detector. Similar to literature data, it is observed that TOF decreases with an increase in D. The aim of the present study is to correlate this observation with the modifications of the surface elementary steps of two kinetic models, M1 and M2, developed previously for D ≲ 0.6 . The elementary steps considered are (1) adsorption of CO as a linear CO species (denoted by L and involved in models M1 and M2) and as a bridged CO species (denoted by B); (2) the oxidation by O 2 of the adsorbed CO species; and (3) the reduction by CO of the strongly adsorbed oxygen species (denoted by O sads) involved in model M2. It is shown that D has no significant impact on the heat of adsorption of the L CO species and on the mechanism of the reduction of O sads species by CO. For oxidation of the L CO species by O 2, it is shown that there is an induction period in the CO 2 production for D ≳ 0.60 not observed for a lower dispersion. This is explained by considering that the rate of formation of the Pt sites, which adsorb O 2 during oxidation, depends on D. These sites, which represent a small fraction of the Pt 0 sites, are associated with the B CO species. It is shown that the heat of adsorption of the B CO species increases with an increase in D. It is suggested that it is the removal of the B CO species (by desorption and oxidation) that controls the induction duration.

CO Oxidation Kinetics over a Nanostructured Cu0.1Ce0.9O2-yCatalyst: A CO/O2Concentration Cycling Study

Feed composition cycling as a transient kinetic technique provides detailed information about elementary steps in CO oxidation over a nanostructured Cu0.1Ce0.9O2−y catalyst. This catalyst has a great potential as a future PROX reactor catalyst since it has great oxygen storage capacity as well as high reoxidation rate under oxygen rich conditions.

Kinetics, simulation and insights for CO selective oxidation in fuel cell applications

The kinetics of CO preferential oxidation (PROX) was studied to evaluate various rate expressions and to simulate the performance the CO oxidation step of a methanol fuel processor for fuel cell applications. The reaction was carried out in a micro reactor testing unit using a commercial Engelhard Selectoxo (Pt–Fe/γ-alumina) catalyst and three self-prepared catalysts. Temperature was varied between 100 and 300 °C, and a of range feed rates and compositions were tested. A reaction model in which three reactions (CO oxidation, H2 oxidation and the water gas shift reaction) occur simultaneously was chosen to predict the reactor performance. Using non-linear least squares, empirical power-law type rate expressions were found to fit the experimental data. It was critical to include all three reactions to determine good fitting results. In particular, the reverse water gas shift reaction had an important role when fitting the experimental data precisely and explained the selectivity decrease at higher reaction temperatures. Using this three reaction model, several simulation studies for a commercial PROX reactor were performed. In these simulations, the effect of O2/CO ratio, the effect of water addition, and various non-isothermal modes of operation were evaluated. The results of the simulation were compared with corresponding experimental data and shows good agreement.