Direct CO Oxidation Research Articles

Theoretical insights into CO oxidation activities on CeO2(111) steps

Direct CO oxidation on four types of CeO2(111) surface steps were studied by using density functional theory (DFT) calculations. Our results showed that CO adsorption at step edges can be enhanced through the alignment of C-O and surface Ce-O bonds. Reactivities of various step edges on the CeO2(111) surface were found to be similar to those of the corresponding extended facets (i.e., {110} and {100}). In general, the oxygen sites at step edges with lower coordination numbers can give smaller vacancy formation energies and higher reactivities. In particular, nearly all the O species at the lower position of the step edges exhibit superior catalytic activities than those on the upper position. Moreover, it was found that the unusual decrease of the Ce 4f orbital energy level after CO adsorption on the Type II* step can contribute to its excellent catalytic activity towards CO oxidation, which is caused by the synergistic effect of structural and electronic distortions on the low-coordinated surface Ce5c at the step edge. Our results disclosed the unique role CeO2(111) steps play that influences the reactant adsorption, activation barriers, product desorption and thus the CO catalytic oxidation activities overall. These findings may inspire persistent research interests on the structure-reactivity relationships over surface defect sites on rare-earth oxide catalysts.

The Regulation of O2 Spin State and Direct Oxidation of CO at Room Temperature Using Triboelectric Plasma by Harvesting Mechanical Energy.

Oxidation reactions play a critical role in processes involving energy utilization, chemical conversion, and pollutant elimination. However, due to its spin-forbidden nature, the reaction of molecular dioxygen (O2) with a substrate is difficult under mild conditions. Herein, we describe a system that activates O2 via the direct modulation of its spin state by mechanical energy-induced triboelectric corona plasma, enabling the CO oxidation reaction under normal temperature and pressure. Under optimized reaction conditions, the activity was 7.2 μmol h−1, and the energy consumption per mole CO was 4.2 MJ. The results of kinetic isotope effect, colorimetry, and density functional theory calculation studies demonstrated that electrons generated in the triboelectric plasma were directly injected into the antibonding orbital of O2 to form highly reactive negative ions O2−, which effectively promoted the rate-limiting step of O2 dissociation. The barrier of the reaction of O2− ions and CO molecular was 3.4 eV lower than that of O2 and CO molecular. This work provides an effective strategy for using renewable and green mechanical energy to realize spin-forbidden reactions of small molecules.

Active copper species of co-precipitated copper-ceria catalysts in the CO-PROX reaction: An in situ XANES and DRIFTS study

Properties of co-precipitated CuO/CeO2 catalysts with nominal content of 1–3 wt.% copper were investigated by EDS, XRD, BET, STEM, EDS map, H2-TPR, in situ DRIFTS and in situ XANES. By comparing results of DRIFTS, XANES and catalytic performance in the CO-PROX reaction, it was evidenced a relationship between the catalytic behavior and the copper oxidation state of the materials. In situ DRIFTS and XANES analyses shed light on the oxidation state of active copper species involved in the CO-PROX reaction between 120 °C and 200 °C. The investigated catalysts present a balance between Cu° and Cu+ species under PROX conditions and the Cu+ is the main species liable to adsorb CO. Furthermore, our studies indicated that the PROX reaction on CuO/CeO2 is impacted by the nature of Cu species, which in turn may vary with reaction temperature, depending on the Cu content. From these results, we propose a new mechanism of PROX reaction on CuCe catalysts, including Cu°/Cu+ and Ce3+/Ce4+ redox pairs and direct CO oxidation on Cu°.

Atom-specific activation in CO oxidation.

We report on atom-specific activation of CO oxidation on Ru(0001) via resonant X-ray excitation. We show that resonant 1s core-level excitation of atomically adsorbed oxygen in the co-adsorbed phase of CO and oxygen directly drives CO oxidation. We separate this direct resonant channel from indirectly driven oxidation via X-ray induced substrate heating. Based on density functional theory calculations, we identify the valence-excited state created by the Auger decay as the driving electronic state for direct CO oxidation. We utilized the fresh-slice multi-pulse mode at the Linac Coherent Light Source that provided time-overlapped and 30 fs delayed pairs of soft X-ray pulses and discuss the prospects of femtosecond X-ray pump X-ray spectroscopy probe, as well as X-ray two-pulse correlation measurements for fundamental investigations of chemical reactions via selective X-ray excitation.

Density Functional Theory Investigation of the Role of Cocatalytic Water in the Water Gas Shift Reaction over Anatase TiO2 (101)

The water gas shift reaction (WGS) mechanism on the anatase TiO2 (101) surface has been investigated using periodic density functional theory. Reaction energies and activation energy barriers for direct CO oxidation and associative pathways with carboxyl and formate intermediates were calculated and compared. Unassisted by water, the formation of formate was infeasible due to the difficulty in transferring hydrogen from the anatase surface to hydrogenate CO, while the carboxyl mechanism had the lowest apparent activation energy barrier among the three. Coadsorbed water species were found to be effective hydrogen transfer cocatalysts, with molecularly physisorbed water acting as a hydrogen donor and dissociatively chemisorbed water acting as a hydrogen acceptor. Using water as the hydrogen donor, CO hydrogenation became competitive with the carboxyl mechanism, indicating that the reactivity of water could change the relative competitiveness of different WGS pathways. These results suggest that adsorbed wat...

Isolating the impact of CO concentration in syngas mixtures on SOFC performance via internal reforming and direct oxidation

One of the distinct advantages of solid oxide fuel cells (SOFCs) is their ability to directly oxidize CO in addition to H2, which allows them to be run on syngas mixtures. However, membrane-electrode-assembly (MEA) models typically neglect CO electrochemistry in the presence of H2 and H2O, assuming that the water–gas-shift reaction proceeds faster than sluggish CO electro-oxidation. In this paper, however, we demonstrate with a comprehensive 1D-MEA model that CO electro-oxidation cannot be neglected in syngas mixtures, particularly at high current densities for high CO-content syngas. We first demonstrate that incoming CO is not all shifted to form H2 before reaching the triple-phase boundary, as previously assumed, due to the equilibrium limitation of the water–gas-shift reaction at 800 °C. Furthermore, we confirm that direct oxidation of CO contributes non-negligible current relative to H2 at high anode overpotentials in syngas mixtures. Together these results show that CO electro-oxidation plays an important role in SOFC performance not only via water–gas-shift reforming, but also via direct oxidation even when H2 is present. This work suggests that accurate models for both surface reforming and direct electro-oxidation of CO in SOFC anodes must be included in order to capture performance when using syngas mixtures.

Assessment of finite volume modeling approaches for intermediate temperature Solid Oxide Fuel Cells working with CO-rich syngas fuels

The application of Solid Oxide Fuel Cells (SOFCs) to a wide variety of syngas fuels, as those generated from coal or biomass gasification, may represent an important opportunity for the improvement of electric efficiency and for emission reduction in the electric sector.The paper presents an assessment of the impact on SOFC performance prediction of the effect of the combined electrochemical oxidation of CO and H2, proposing additional analyses to delve into the effects of different SMR and WGS kinetics models, along with the risk of carbon deposition. The work is based on a finite-volume model for SOFC simulation, updated to examine two different electrochemical reaction approaches where (i) the first considers only the electrochemical conversion of H2 and neglects the possibility of a direct CO oxidation at electrode surface; (ii) the second calculates the current contribution of both H2 and CO oxidation. The two models are compared against different test cases with low and high-CO syngas, representative of different primary fuels, discussing the relative proportion of H2 and CO-related current densities depending on the inlet fuel composition. A sensitivity analysis is carried out on the electrochemical model, focusing on the activation overpotentials. We show that the resulting enhanced H2–CO electrochemical model allows a better understanding of the fuel composition and H2/CO ratios effect on the cell performances.

Direct CO oxidation by lattice oxygen on the SnO2(110) surface: a DFT study

As a noble-metal-free catalyst for CO oxidation, SnO2 has sparked worldwide interest owing to its highly reactive lattice oxygen atoms and low cost. The current density functional theory (DFT) results demonstrate the process of CO oxidation by lattice oxygen on the SnO2(110) surface and the recovery of the reduced surface by O2. It is found that CO can be easily oxidized on the SnO2(110) surface following the Mars-van Krevelen mechanism. The adsorbed oxygen turns into various oxygen species by transferring electron(s) to the chemisorbed oxygen, which is only found on the partially reduced SnO2-x surface, but not on the perfect SnO2(110) surface: O2(gas) ↔ O2(ad) ↔ O2(-)(ad) ↔ O2(2-)(ad) ↔ O(2-)(lattice) + O(-)(ad). The calculated stretching frequencies would help to distinguish the various adsorbed species observed in experiment and of course help in the assignment of vibrational modes in the experimental spectra.

Kinetics of the water-gas shift reaction over Rh/Al2O3 catalysts

The kinetics of the water-gas shift (WGS) reaction over Rh/Al2O3 catalyst is studied experimentally and numerically. Using the experimentally determined conversion in WGS, reverse WGS, and preferential oxidation of CO over a catalytically coated disk and over a honeycomb monolith, a thermodynamically consistent multi-step reaction mechanism with the associated rate expressions was developed. Both the experimental configurations were numerically simulated coupling models for the flow field with this heterogeneous reaction mechanism. The main reaction path for CO2 formation on this catalyst is concluded to be the direct oxidation of CO with O species at high temperatures, whereas the formation of the carboxyl (COOH) group is significant at temperature below 600°C. The reaction kinetics reproduced the experimental observations, also for the subsystems of hydrogen and CO oxidation.

Critical role of water in the direct oxidation of CO and hydrocarbons in diesel exhaust after treatment catalysis

CO and C3H6 oxidation have been carried out in the absence and presence of water over a Pd/Al2O3 catalyst. It is clear that water promotes CO and, as a consequence, C3H6 oxidation takes place at much lower temperatures compared with the dry feed. The significant increase in the catalyst's activity with respect to CO oxidation is not simply associated with changes in surface concentration as a result of competitive adsorption effects. Utilising 18O2 as the reactant allows the pathways whereby the oxidation due to gaseous dioxygen and where the water activates the CO and C3H6 to be distinguished. In the presence of water, the predominant pathway is via water activation with C16O2 and C16O18O being the major species formed and oxidation with dioxygen plays a secondary role. The importance of water activation is further supported by the significant decrease in its effect when using D2O versus H2O.

A DFT + U study of CO oxidation at CeO2(110) and (111) surfaces with oxygen vacancies

Density functional theory calculations corrected by on-site Coulomb interaction have been conducted to illuminate the effect of O vacancies on the surface reactivities toward direct CO oxidation at CeO2(110) and (111). CO adsorption at and further reaction with various surface O at the vacancy were systematically calculated, and extensive geometric and electronic analyses were also performed to understand the calculation results. It has been shown that though the existence of O vacancy can reduce the activities of neighboring O as the barriers of elementary steps are generally higher than those at stoichiometric surfaces, the relative preference for CO2 desorption is also increased due to the missing of nearby O for carbonate formation. Moreover, our calculation results also showed that the characteristic localized 4f orbitals/electrons of Ce still play the key role in determining the surface reactivities as the vacancy induced localized electrons can hinder their further occurrence and the corresponding reactions.

Catalytic DBD plasma reactor for CO oxidation by in situ N2O decomposition

Carbon monoxide oxidation by nitrous oxide was carried out in a dielectric barrier discharge reactor operated under ambient conditions. It was observed that nonthermal plasma is effective in direct decomposition of N2O into nitrogen and atomic oxygen that promoted CO oxidation. Interesting observation is that plasma reactor has not shown significant activity for direct CO oxidation, whereas, presence of N2O improved the conversion of CO. Typical results indicated that the addition of ceria based catalysts improved CO conversion significantly, which was assigned due to adsorption/stabilization of atomic oxygen on the surface of the catalyst. Optimization of various parameters such as residence time of the gas, initial concentrations of CO, N2O and catalyst was done to achieve the best efficiency of the reactor. It was observed that conversion decreased with increasing flow rate of the gas and CO and N2O initial concentrations. Typical results indicated the best results at high residence of the gas time and low concentration of the reactants. Addition of catalyst improved the performance of the reactor due to synergetic effect between plasma excitation and catalytic action. Plasma reactor integrated with NiO/CeO2 catalyst showed the best CO2 selectivity.

A DFT+U study of the lattice oxygen reactivity toward direct CO oxidation on the CeO2(111) and (110) surfaces

Density functional theory calculations corrected by on-site Coulomb interaction have been carried out to track down the lattice oxygen reactivity of CeO(2)(111) and (110) surfaces in direct oxidation of a single CO. The possible elementary steps in CO adsorption and subsequent reactions with lattice oxygen were systematically studied. From calculated energetics, we determined that the lattice oxygen of the (110) surface is more reactive than that of the (111) surface. By calculating the reaction pathways leading to different final products, we found that the formation of carbonate species is competitive to CO(2) formation and desorption, and such an effect could be more significant at CeO(2)(110) compared to CeO(2)(111). More importantly, it has also been found that electron localization at the characteristic 4f orbital of Ce, directly determined by subtle structural relaxation, can give rise to a unique scenario of the overall reaction coordinates. These results may bring us one step ahead toward the comprehensive understanding of catalytic performance of CeO(2)-based materials.

Deep reduction behavior of iron oxide and its effect on direct CO oxidation

Reduction of metal oxide oxygen carrier has been attractive for direct CO oxidation and CO2 separation. To investigate the reduction behaviors of iron oxide prepared by supporting Fe2O3 on γ-Al2O3 and its effect on CO oxidation, fluidized-bed combustion experiments, thermogravimetric analyzer (TGA) experiments, and density functional theory (DFT) calculations were carried out. Gas yield (γCO2) increases significantly with the increase of temperature from 693K to 1203K, while carbon deposition decreases with the increase of temperature from 743K to 1203K, where temperature is a very important factor for CO oxidation by iron oxide. Further, it were quantitatively detected that the interaction between CO and Fe2O3, breakage of O–Fe bonds and formation of new C–O bonds, and effect of reduction degree were quantitatively detected. Based on adsorptions under different temperatures and reducing processes from Fe3+ into Fe2+, Fe+ and then into Fe, it was found that Fe2+→Fe+ was the reaction-controlling step and the high oxidation state of iron is active for CO oxidation, where efficient partial reduction of Fe2O3 into FeO rather than complete reduction into iron may be more energy-saving for CO oxidation.

Direct CO Oxidation by Lattice Oxygen on Zr-Doped Ceria Surfaces

Systematic density-functional calculations have been performed to address an important issue for CO oxidation on redox ceria: the role of lattice oxygen. One major findings is that CO easily grasps one lattice oxygen atom to form CO2 − and CO2 on CeO2 (111) and Ce0.75Zr0.25O2 (111) with small activation energies. Zr dopants facilitate the reduction of Ce+4 to Ce+3 and hence weaken the Ce–O bonds, which benefit the direct formation and release of CO2. Zr dopants facilitate the reduction of Ce(IV) to Ce(III) and hence weaken the Ce–O bonds, which benefit the direct oxidation of CO on Zr-doped ceria.

Influence of NO on the Reduction of NO2 with CO over Pt/SiO2 in the Presence of O2

AbstractReduction of NO2 with CO in the presence of NO and excess oxygen, a model mixture for flue gas, over a 0.1% Pt/SiO2 catalyst was studied. The related reaction mechanisms, such as oxidation of CO and NO, were discussed. It was found that there was a narrow temperature window (180–190 °C) for the reduction of NO2 by CO. When the temperature was lower than the lower limit of the window, the reduction hardly occurred, while when the temperature was higher than the upper limit of the window, the direct oxidation of CO by O2 occurred and thereby NO2 could not be effectively reduced by CO. The presence of NO shifted the window to higher temperatures owing to the inhibition effect of NO on the activation of O2 on Pt, which made it possible to reduce NO2 by CO in flue gas.

CO Coverage/Oxidation Correlated with PtRu Electrocatalyst Particle Morphology in 0.3 M Methanol by In Situ XAS

In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Pt and Ru K edges, were carried out on three different carbon-supported PtRu electrocatalysts in an electrochemical cell in with methanol. The CO and OH adsorbate coverage on Pt and Ru were determined as a function of the applied potential via the novel delta XANES technique, and the particle morphology was determined from the EXAFS and a modeling technique. Both the bifunctional and direct CO oxidation mechanisms, the latter enhanced by electronic ligand effects, were evident for all three electrocatalysts; however, the dominant mechanism depended critically on the particle size and morphology. Both the Ru island size and overall cluster size had a very large effect on the CO oxidation mechanism and activation of water, with the bifunctional mechanism dominating for more monodispersed Ru islands, and the direct surface ligand effect dominating in the presence of larger Ru islands.

Methanol steam reforming over ex-hydrotalcite Cu–Zn–Al catalysts

Cu–ZnO–Al 2O 3 catalysts have been prepared via hydrotalcite-like hydroxycarbonate precursors and tested in the oxidative steam reforming of methanol (MOSR) to produce hydrogen with poor CO content. The surface chemistry of the catalysts has been investigated by IR spectroscopy of low temperature CO adsorption. The aim was to gain indications on the optimal catalyst composition for MOSR and on the role of the CO into CO 2 conversion step. The conclusions are that the catalytic activity is almost independent on the copper content in the range 5–12% (atomic metal content). The direct oxidation of CO to CO 2 by Cu 2+ ions is likely a key step both for MOSR and water gas shift over these catalysts.

Novel mechanism for oxidation of CO by Fe 2O 3 clusters

The oxidation of CO by Fe 2O 3 clusters has been studied using a first principles gradient corrected density functional approach. It is shown that the direct oxidation of CO via surface oxygen atoms is feasible with a low barrier of 0.4 eV. The present studies also suggest a new indirect mechanism for CO oxidation. Here, the first CO molecule adsorbed on a surface Fe site breaks one of the FeO bonds and a subsequent CO molecule is oxidized by the less coordinated O atom in an almost barrierless reaction pathway. Subsequent attachment of CO and O 2 to Fe sites results in the formation of a CO 3 complex where the heat of formation is enough to allow CO 2 to leave the surface reverting the system back to Fe 2O 3. The role of charge transfer and the nature of the transition states are highlighted. It is shown that small clusters are ferromagnetic indicating that super-exchange interactions of the antiferromagnetic bulk are modified at the reduced sizes.

Carbon monoxide detection of chemisorbed oxygen in coal and other carbonaceous materials

The oxidation of carbon monoxide by mildly oxidized and devolatilized coal samples was studied thermogravimetrically. The oxidation was attributed to oxygen chemisorbed on inorganic components of the coals. The reaction of CO with pyrite producing carbonyl sulphide, OCS, accompanied the oxidation. A mechanism for CO oxidation is proposed in which active oxygen chemisorbed on the inorganic components of the coal directly oxidized CO to CO 2, and facilitates the chemisorption of CO on the coal as carbonate. A factor, α = ( 11 14 ) [1 − ( W n W c )] , was derived where W n is the sample weight loss not attributed to OCS formation, and W c is the estimated weight of evolved CO 2. This quantity is proportional to the fraction of CO 2 produced by the direct oxidation of CO, and was used to compare the coal samples studied. Samples of an Illinois No. 5 coal yielded average α values of 0.7 and those of an Illinois No. 6 coal yielded values of 0.6, indicating that in these cases, the majority of CO 2 produced came from the direct oxidation of CO. The results obtained for the coal samples are compared with a selection of carbonaceous samples for which the proposed mechanism does not apply.