Rate Of CO Oxidation Research Articles

Effect of surface Fe-S hybrid structure on the activity of the perfect and reduced α-Fe2O3(001) for chemical looping combustion

Sulfurization of the gradually reduced Fe2O3 surfaces is inevitable while Fe2O3 is used as an oxygen carrier (OC) for coal chemical looping combustion (CLC), which will result in formation of Fe-S hybrid structure on the surfaces. The Fe-S hybrid structure will directly alter the reactivity of the surfaces. Therefore, detailed properties of Fe-S hybrid structure over the perfect and reduced Fe2O3(001) surfaces, and its effect on the interfacial interactions, including CO oxidization and decomposition on the surfaces, were investigated by using density functional theory (DFT) calculations. The S atom prefers to chemically bind to Fe site with electron transfer from the surfaces to the S atom, and a deeper reduction of Fe2O3(001) leads to an increasing interaction between S and Fe. The formation of Fe-S hybrid structure alters the electronic properties of the gradually reduced Fe2O3(001) surfaces, promoting CO oxidation on the surfaces ranging from Fe2O3 to FeO, but depressing carbon deposition on the surfaces ranging from FeO to Fe. The sulfurized FeO acts as a watershed to realize relatively high CO oxidation rate and low carbon deposition. Results provided a fundamental understanding for controlling and optimizing the CLC processes.

Preferential oxidation of CO in H2 on Cu and Cu/CeOx catalysts studied by in situ UV–Vis and mass spectrometry and DFT

Preferential oxidation of CO in H2 was studied by in situ ultraviolet–visible (UV–Vis) and mass spectrometry on flat model Cu and Cu/CeOx catalysts. The experimental findings were interpreted and compared with the results from density functional theory (DFT) calculations of the adsorption and activation energies for the essential reaction steps on Cu(1 1 1). It was found that oxidation of CO preferentially takes place on Cu(0) and that no significant H2 oxidation took place under any of the investigated conditions. The presence of CeOx accelerates Cu(0)-oxidation which leads to catalyst deactivation. In contrast, CeOx promotes the CO oxidation rate on catalysts that were already oxidized to CuOx. The coexistence of CO and H2 is important to sustain the stability of metallic Cu and thereby a high rate of CO2 formation. In pure CO/O2 gas, the metallic phase can only be maintained as long as full O2 conversion is reached. In pure H2/O2, Cu is always partly but never fully oxidized, suggesting that a passivating surface layer is formed. This is also the case for H2 rich gas mixtures with small amounts of CO and O2. The most active surface termination, Cu(0), can therefore not be maintained under the industrially most interesting reaction condition where full conversion of trace amounts of CO in H2 is required. DFT calculations predict that the dissociative H2 adsorption is a key limiting step for hydrogen oxidation on the Cu(1 1 1) surface, especially when the low sticking coefficient is taken into account.

Steric effect in CO oxidation on Pt(111).

We present experimental evidence showing that the rate of CO oxidation on Pt(111) depends strongly on the geometry of an incident O2 molecule. The O2 sticking probability and the CO2 production rate on CO-covered Pt(111) surfaces during the alignment-controlled O2 beam irradiation were simultaneously monitored at a surface temperature of 330 K. The results indicate that, at the translational energy of 0.1-0.2 eV and at the CO coverage of <0.4 monolayers, both O2 adsorption and CO oxidation proceed exclusively when the O2 molecular axis is nearly parallel to the surface.

Investigation of the Thermodynamic Properties of Surface Ceria and Ceria–Zirconia Solid Solution Films Prepared by Atomic Layer Deposition on Al2O3

The properties of 20 wt % CeO2 and 21 wt % Ce0.5Zr0.5O2 films, deposited onto a γ-Al2O3 by Atomic Layer Deposition (ALD), were compared to bulk Ce0.5Zr0.5O2 and γ-Al2O3-supported samples on which 20 wt % CeO2 or 21 wt % CeO2–ZrO2 were deposited by impregnation. Following calcination to 1073 K, the ALD-prepared catalysts showed much lower XRD peak intensities, implying that these samples existed as thin films, rather than larger crystallites. Following the addition of 1 wt % Pd to each of the supports, the ALD-prepared samples exhibited much higher rates for CO oxidation due to better interfacial contact between the Pd and ceria-containing phases. The redox properties of the ALD samples and bulk Ce0.5Zr0.5O2 were measured by determining the oxidation state of the ceria as a function of the H2:H2O ratio using flow titration and coulometric titration. The 20 wt % CeO2 ALD film exhibited similar thermodynamics to that measured previously for a sample prepared by impregnation. However, the sample with 21 wt % Ce0.5Zr0.5O2 on γ-Al2O3 reduced at a much higher P O 2 and showed evidence for transition between the Ce0.5Zr0.5O2 and Ce0.5Zr0.5O1.75 phases.

Binding of ibuprofen to human hemoglobin: elucidation of their molecular recognition by spectroscopy, calorimetry, and molecular modeling techniques

Ibuprofen, used for the treatment of acute and chronic pain, osteoarthritis, rheumatoid arthritis, and related conditions has ample affinity to globular proteins. Here we have explored this fundamental study pertaining to the interaction of ibuprofen with human hemoglobin (HHb), using multispectroscopic, calorimetric, and molecular modeling techniques to gain insights into molecular aspects of binding mechanism. Ibuprofen-induced graded decrease in absorption spectra indicates protein disruption along with sedimentation of HHb particle. Red shifting of absorption peak at 195 nm indicates alteration in the secondary structure of HHb upon interaction with ibuprofen. Flouremetric and isothermal titration calorimetric (ITC) studies suggested one binding site in HHb for ibuprofen at 298.15 K. However, with increase in temperature, ITC revealed increasing number of binding sites. The negative values of Gibbs energy change (ΔG0) and enthalpy change (ΔH0) along with positive value of entropy change (ΔS0) strongly suggest that it is entropy-driven spontaneous exothermic reaction. Moreover, hydrophobic interaction, hydrogen bonding, and π–π interaction play major role in this binding process as evidenced from ANS (8-anilino-1-napthalenesulphonic acid), sucrose binding, and molecular modeling studies. The interaction impacts on structural integrity and functional aspects of HHb as confirmed by CD spectroscopy, increased free iron release, increased rate of co-oxidation and decreased rate of esterase activity. These findings suggest us to conclude that ibuprofen upon interaction perturbs both structural and functional aspects of HHb.

Kinetic Coupling among Metal and Oxide Phases during CO Oxidation on Partially Reduced PdO(101): Influence of Gas-Phase Composition

We used direct-rate measurements and reflection absorption infrared spectroscopy (RAIRS) to investigate CO oxidation on a PdO(101) film at 450 K and focused on characterizing how the gas-phase composition (O2 + CO) influences the reaction kinetics and the coupling between the reaction and the surface phases that develop. We find that chemisorbed O atoms on Pd(111) are intrinsically more reactive than PdO(101) and that CO oxidation rates can be as much as 2–3 times higher on Pd(111) compared with PdO(101) for the conditions studied. Using RAIRS, we show that reduction of PdO(101) by CO produces metallic Pd(111) domains at 450 K and identified several rate processes that couple the PdO(101) and Pd(111) phases during reaction, including the formation of Pd(111) domains from O vacancies on PdO(101), oxygen transfer from the oxide to the metal to generate chemisorbed O atoms, and the healing of surface oxygen vacancies on PdO(101) via O atom migration from the bulk oxide as well as O2 dissociative adsorption. ...

A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic cycles of CO oxidation

CO oxidation over Pd/WO3 films prepared on a glass substrate has been examined at the substrate temperature of 150 – 250 °C and pressures less than 1 Torr with a stoichiometric mixture of CO and O2. Under the given reaction condition, the chemical states of the Pd/WO3 film gradually change into the most catalytically active form with the highest saturation reaction rate regardless of the initial oxidation states. The measured CO oxidation rate over the Pd/WO3 is strongly dependent on the chemical states of Pd and W. Either metallic Pd or fully oxidized PdO phase is not as catalytically active as the active form with mixed metallic Pd and thin PdO layers supported on WO3 with partially reduced W5+ state which is spontaneously obtained during the catalytic reaction cycles. Our results indicate that the facile oxygen transfer between Pd and WO3 layers not only facilitate the spontaneous changes into the active form, but also act as a promotional role in CO oxidation over the Pd layer.

An evaluation of myeloperoxidase-mediated bio-activation of NSAIDs in promyelocytic leukemia (HL-60) cells for potential cytotoxic selectivity

Several lines of evidence have pointed towards the potential therapeutic benefit of NSAIDs in cancer therapy. In this study, we have investigated the acute bio-activation of NSAIDs and their metabolites via myeloperoxidase (MPO), a highly-expressed peroxidase enzyme in acute myeloid leukemia. As bio-activation involves the formation of reactive metabolites, we hypothesized that NSAIDs which produced reactive metabolites would be correlated with leukemia cell toxicity. We tested the enzymatic peroxidation of three NSAIDs, namely diclofenac, indomethacin, and naproxen in comparison with their hepatic metabolites, 4′- hydroxydiclofenac (4′-OHD), 5-hydroxydiclofenac (5-OHD), O-desmethyl-N-deschlorobenzoylindomethacin (DMBI), O-desmethylindomethacin (DMI) and O-desmethylnaproxen (ODN). Firstly, we used purified peroxidases in kinetic UV–vis kinetic spectrophotometry, and electron paramagnetic resonance (EPR) experiments to determine oxidation of ascorbic acid and glutathione (GSH), respectively. We then used HL-60 cells, as a model of acute myelogenous leukemia to carry out trypan blue exclusion, cellular ATP analysis, mitochondrial membrane potential (MMP) and cytofluorometric GSH assays. Our results present evidence that diclofenac, 4′-OHD, 5-OHD, DMBI and DMI demonstrated significant cytotoxic effect in the leukemic cells through oxidation by intracellular MPO. In the same vein, only diclofenac and its two metabolites caused a significant drop in the MMP and cellular ATP level; however, the cell death induced by indomethacin metabolites reflected a subtle effect on MMP or GSH content. Interestingly, only diclofenac and 4′-OHD (and not 5- OHD) caused a significant drop in HL-60 cells’ GSH content. Among diclofenac compounds, only 4′-OHD also generated GS radical and caused a significant increase in ascorbate co-oxidation rate. Lastly, even though ODN also generated GS radical and potently cooxidized ascorbate, it showed no significant cytotoxicity. These results provide evidence of a correlation between acute cytotoxicity and MPO-bioactivated NSAIDs, though this was not correlated for all compounds (e.g., ODN). Further studies are required to determine both the MPO-dependent and MPO-independent mechanisms of cytotoxicity.

Ultrasmall gold nanoparticles confined in zeolite Y: Preparation and activity in CO oxidation

In this study, proton-type zeolites Y (HY) with Al/Si ratio of 0.36 were used as a support for deposition of HAuCl4 (in pH 6 solution) to prepare Au/Y catalysts for CO oxidation at 25°C. The low isoelectric point of HY (∼pH=2) made this support inappropriate for loading negatively charged gold complexes, and caused the generation of large gold particles with low catalytic activity. We found that through a Na+ pretreatment of HY to form H(Na)Y, the surface charge can be reversed to positive, thus favoring the deposition. The resulting Au/H(Na)Y, without any high temperature treatment, gave excellent activity in CO oxidation comparable to Au/TiO2. The CO oxidation rate of 0.21mol CO/mol Au∙s remained stable at 25°C. HRTEM images of the catalyst show highly dispersed gold nanoparticles ∼1nm confined in the nanocage of H(Na)Y. XPS spectra show high proportion of Au(III) associated with Au-O. The weak Au-O in Au/H(Na)Y is found responsible for the easy reduction in TPR. For comparison, the other alternative procedures of gold deposition were also tried to reverse the surface charge of HY by decreasing the pH of solution. However, high contents of residual Cl in the nano-gold catalyst lead to almost no activity in CO oxidation.

CO oxidation and O2 removal on meteoric material in Venus’ atmosphere

The heterogeneous oxidation of CO by O2 on olivine, Fe sulfate and Fe oxide particles was studied using a flow tube apparatus between 300 and 680K. These particles were chosen as possible analogues of unablated cosmic dust and meteoric smoke in Venus’ atmosphere. On olivine and Fe oxides, the rate of CO oxidation to CO2 only becomes significant above 450K. For iron sulfates, CO2 production was not observed until these dust analogues had decomposed into iron oxides at ∼ 540K. The CO oxidation rate increases significantly with a higher Fe content in the dust, implying that oxidation occurs through Fe active sites (no reaction was observed on Mg2SiO4). The oxidation kinetics can be explained by CO reacting with chemi-sorbed O2 through an Eley–Rideal mechanism, which is supported by electronic structure calculations. Uptake coefficients were measured from 450 to 680K, yielding: log10(γ (CO on MgFeSiO4))=(2.9 ± 0.1)× 10-3T(K)–(8.2 ± 0.1); log10(γ (CO on Fe2SiO4))=(2.3 ± 0.3)× 10-3T(K) –(7.7 ± 0.2); log10(γ (CO on FeOOH/Fe2O3))=(5.6 ± 0.8)× 10-3T(K)–(9.3 ± 0.4). A 1-D atmospheric model of Venus was then constructed to explore the role of heterogeneous oxidation. The cosmic dust input to Venus, mostly originating from Jupiter Family Comets, is around 32 tonnes per Earth day. A chemical ablation model was used to show that ∼34% of this incoming mass ablates, forming meteoric smoke particles which, together with unablated dust particles, provide a significant surface for the heterogeneous oxidation of CO to CO2 in Venus’ troposphere. This process should cause almost complete removal of O2 below 40km, but have a relatively small impact on the CO mixing ratio (since CO is in large excess over O2). Theoretical quantum calculations indicate that the gas-phase oxidation of CO by SO2 in the lower troposphere is not competitive with the heterogeneous oxidation of CO. Finally, the substantial number density of meteoric smoke particles predicted to occur above the cloud tops may facilitate the low temperature heterogeneous chemistry of other species.

Mechanistic Aspects of Wet and Dry CO Oxidation on Co3O4 Nanorod Surfaces: A NAP-UPS Study.

Catalytic activity, electronic structure, and the mechanistic aspects of Co3O4 nanorod (NR) surfaces have been explored for CO oxidation in dry and wet atmosphere using near-ambient pressure ultraviolet photoelectron spectroscopy. Presence of water with CO + O2 plummets the catalytic activity because of the change in the electronic nature from predominantly oxide (without water in feed) to a Co3O4 surface covered by a few intermediates. However, at ≥375 K, the Co3O4 surface recovers and regains the oxidation activity, at least partially, even in the presence of water. This is fully supported by the changes observed in the work function of Co3O4 under wet (H2O + CO + O2) conditions compared with dry (CO + O2) conditions. This study focuses on the comparative CO oxidation rate on Co3O4 NR surfaces and highlights the changes in the electronic structure that occur in the catalyst during the CO oxidation reaction.

Improved CO Oxidation Activity of 3DOM Pr-Doped Ceria Catalysts: Something Other Than an Ordered Macroporous Structure

It is demonstrated that the synthesis procedure for preparing three-dimensionally ordered macroporous (3DOM) Pr-doped ceria catalysts using a polymethylmethacrylate (PMMA) template not only affects the porous structure, but also the chemistry of the ceria surface. The PMMA template does not affect the crystalline features (type of phases, crystallite size, and cell parameter) of Pr-doped ceria, Ce and Pr location into the particles, and the bulk reduction of the Ce-Pr mixed oxide catalysts. On the contrary, the utilization of the PMMA template improves both the porosity and surface redox properties. 3DOM Ce-Pr mixed oxide catalysts combine micro, meso, and macropores, the most area being in the macropore range, while a reference unshaped catalyst presents poor porosity in all ranges. However, the catalyzed CO oxidation rates do not correlate with the surface area of the catalysts (neither micro nor meso/macro). The Ce-Pr-3DOM catalyst also presents improved surface reducibility with regards to the counterpart reference material prepared without the template, and improved redox behavior under reaction conditions; that is, it has a higher area and this area is reduced and reoxidized more easily. X-ray photoelectron spectroscopy analysis evidences that this is mainly attributed to praseodymium cations, which accomplish redox cycles more easily than cerium cations.

Ag supported on meso-structured SiO2 with different morphologies for CO oxidation: On the inherent factors influencing the activity of Ag catalysts

With the objective to prepare better CO oxidation catalysts and explore the inherent factors to determine the activity of supported Ag catalysts, KCC-1, SBA-15 and MCM-41, a series of mesoporous SiO2 with different morphologies, have been synthesized and used as supports for Ag to prepare catalysts (7% Ag/KCC-1, 7% Ag/SBA-15 and 7% Ag/MCM-41) for CO oxidation. It is found that the morphologies of the SiO2 supports influence the dispersion of the supported Ag species, thus resulting in catalysts with different Ag grain sizes and active metallic surface areas, as proved by XRD and H2 adsorption-desorption results. With fibrous KCC-1 silica spheres as the support, the mobilization and aggregation of the Ag particles can be effectively hindered by the open access surface mesopores. Therefore, compared with SBA-15 and MCM-41, Ag nanoparticles with much smaller grain sizes have been achieved, thus resulting in a catalyst with significantly improved activity. Furthermore, a linear relationship has been found between the differential CO oxidation rates and the Ag grain size. Therefore, it is concluded that the active Ag metal surface area is the determining factor for the activity of the silica supported Ag catalysts.

UV-LED Photocatalytic Oxidation of CO over the Pd/TiO2 Catalysts Synthesized by the Decomposition of Pd(acac)2

The Pd nanoparticles were deposited on the TiO2 surface via the thermal decomposition of palladium acetylacetonate (Pd(acac)2). The catalytic activity of Pd/TiO2 catalysts was investigated in the CO oxidation under ambient conditions both in the absence and presence of UV-LED irradiation. The effect of additional UV pretreatment of the catalyst on the activity in CO oxidation was also studied. The rate of CO oxidation under UV irradiation was 3.5 times higher than the rate of dark catalytic oxidation. The dark catalytic and photocatalytic oxidation rates were monotonically increased as the Pd content was increased up to 2 wt.%. A small size and narrow size distribution of Pd nanoparticles on the TiO2 surface were the reasons for a high activity in CO oxidation. Al2O3 and SiO2 were also employed for the Pd deposition to investigate the effect of semiconducting support. Pd/TiO2 had much higher CO oxidation rate under UV irradiation than the Pd/Al2O3 and Pd/SiO2 catalysts, which do not have photocatalytic activity. It indicates that in the case of the Pd/TiO2 catalysts UV light energy can be used instead of thermal energy to increase the rate of CO oxidation.

Synthesis and properties of a platinum catalyst supported on plasma chemical silicon carbide

A CO oxidation catalyst based on β–SiC and Pt nanoparticles has been synthesized and studied. The average size of Pt clusters on the surface of the plasma-chemical silicon carbide nanoparticles is close to 4 nm. It has been found that the rate of the CO oxidation reaction at low concentrations (100 mg/m3) in air at room temperature over the catalyst based on platinum and silicon carbide nanoparticles is 60–90 times that over a platinum black-based catalyst with a specific surface area of 30 m2/g. The Pt/SiC catalyst containing 12 wt % Pt has been found to provide the maximum CO oxidation rate.

Influence of Polyoxometalate Protecting Ligands on Catalytic Aerobic Oxidation at the Surfaces of Gold Nanoparticles in Water.

Metal oxide cluster-anion (polyoxometalate, or POM) protecting ligands, [α-PW11O39]7- (1), modify the rates at which 14 nm gold nanoparticles (Au NPs) catalyze an important model reaction, the aerobic (O2) oxidation of CO to CO2 in water. At 20 °C and pH 6.2, the following stoichiometry was observed: CO + O2 + H2O = CO2 + H2O2. After control experiments verified that the H2O2 product was sufficiently stable and did not react with 1 under turnover conditions, quantitative analysis of H2O2 was used to monitor the rates of CO oxidation, which increased linearly with the percent coverage of the Au NPs by 1 (0-64% coverage, with the latter value corresponding to 211 ± 19 surface-bound molecules of 1). X-ray photoelectron spectroscopy of Au NPs protected by a series of POM ligands (K+ salts): 1, the Wells-Dawson ion [α-P2W18O62]6- (2) and the monodefect Keggin anion [α-SiW11O39]8- (3) revealed that binding energies of electrons in the Au 4f7/2 and 4f5/2 atomic orbitals decreased as a linear function of the POM charge and percent coverage of Au NPs, providing a direct correlation between the electronic effects of the POMs bound to the surfaces of the Au NPs and the rates of CO oxidation by O2. Additional data show that this effect is not limited to POMs but occurs, albeit to a lesser extent, when common anions capable of binding to Au-NP surfaces, such as citrate or phosphate, are present.

UV-LED photocatalytic oxidation of carbon monoxide over TiO2 supported with noble metal nanoparticles

The TiO2 catalysts supported with 0.01–4wt% Pt, Pd, or Au were prepared via chemical reduction of inorganic precursors with NaBH4 or their photoreduction in water-ethanol solution under UV irradiation. The samples were characterized by the means of X-ray fluorescence, N2 adsorption, TEM, XPS and UV-Vis spectroscopy and investigated in the processes of dark catalytic and UV-LED photocatalytic oxidation of carbon monoxide at room temperature. UV-LED irradiation of all the synthesized catalysts resulted in an increase in CO oxidation rate compared to the oxidation without UV. Two parallel pathways including thermal catalytic oxidation on metal particles and photocatalytic oxidation by the action of UV light were proposed for the M/TiO2 catalysts.The domed dependence of photocatalytic oxidation rate on the metal content was observed for all the metal deposited catalysts and had a maximum at 1wt% for Pt and 0.05wt% for Pd and Au. The activity of the 1wt% Pt/TiO2 catalyst was 7 and 11 times higher than that of the 0.05wt% Pd- and Au-loaded samples, respectively. For the 1wt% Pt/TiO2 sample, the deposition method via chemical reduction with NaBH4 resulted in a much higher activity than the photoreduction method. The effects of the metal nature, its content and the preparation method on CO photocatalytic oxidation rate are discussed in the terms of metal particle size and charge state based on the data from TEM and XPS analyses.

Catalytic oxidation of solid carbon and carbon monoxide over cerium‐zirconium mixed oxides

A series of cerium‐zirconium mixed oxides was prepared and evaluated in the catalytic oxidation of solid coke with 10 vol % O2 in He at 673 K using a thermogravimetric analyzer. The measured first order rate constant for coke oxidation was proportional to the catalyst loading when the mass ratio of catalyst to coke was low, which enabled the calculation of a surface area specific reaction rate. The validity of the normalization method was confirmed by performing CO oxidation over the cerium‐zirconium mixed oxides in a fixed bed reactor at 573 K. Although there was no correlation between the coke oxidation rate and the oxygen storage capacity or the reducibility of the catalysts, there was an excellent correlation to the CO oxidation rate. Kinetic studies of both coke and CO oxidation suggested an important role of surface lattice oxygen from the catalyst in the two reactions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 725–738, 2017

Role of anionic vacancies in lithium fluoride in the catalytic oxidation of carbon monoxide on the surface of the Au/LiF/Mo(110) system

The oxidation of carbon monoxide molecules on the surface of gold nanoclusters formed on a lithium fluoride film has been investigated by IR spectroscopy and temperature-programed reaction. The CO oxidation rate is much higher in the case of gold clusters produced on the surface of a LiF film enriched with anionic vacancies (F-centers). This is due to the fact that the gold clusters bonded to F-centers of the substrate differ in electronic state from the clusters that are not bonded to F-centers.

Influence of the Copper(II) Oxide Dispersion on its Catalytic Properties in Carbon Monoxide Oxidation: A Comparative Study by Using Two Types of Catalytic Reactors

AbstractThe catalytic properties of copper(II) oxide powders were studied in the CO oxidation reaction by using plug‐flow and flow‐circulation reactors. Data obtained from different catalytic experiments were in good agreement with each other. The specific catalytic rate [molecules cm−2 s−1] increased by a factor of approximately four if the surface area of the CuO powder was reduced from 90 to 8 m2 g−1. A further decrease in the CuO surface area to 1 m2 g−1 resulted in the decrease of the specific CO oxidation rate by more than 20‐fold. These results indicate that CO oxidation is sensitive to the structure of the copper(II) oxide catalyst. The structural sensitivity of CuO powders was discussed in terms of its defect structure and particle morphology.