Mechanism of low-temperature CO oxidation on a model Pd/Fe2O3 catalyst

A model Pd/Fe₂O₃ catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O₂)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe²⁺ (formally Fe₃O₄) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe²⁺) and CO adsorbed on palladium (CO_ads) at the metal–oxide interface.     

In situ FT-IR Spectroscopic Studies on the Mechanism of the Catalytic Oxidation of Carbon Monoxide over Supported Cobalt Catalysts

Three types of supported cobalt catalysts (5% as metal Co loading on SiO₂, Al₂O₃ and TiO₂) were prepared by incipient wetness impregnation with aqueous Co(NO₃)₂·6H₂O solution. Then, all catalysts were calcined in air at 400 °C (assigned as 5Co/Si C400, 5Co/Al C400 and 5Co/Ti C400). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. Adsorption of carbon monoxide (CO) and the co-adsorption of CO/O₂ over cobalt oxide were further tested under in situ FT-IR. The results showed that both 5Co/Si C400 and 5Co/Al C400 had higher activity than 5Co/Ti C400. The T₅₀ (50% conversion) for both 5Co/Si C400 and 5Co/Al C400 was reached at temperatures as low as ambient temperature. According to the in situ FT-IR analysis, the variation in oxidation of CO was interpreted with different mechanisms, i.e., the reaction between adsorbed CO and lattice oxygen of cobalt oxide, and part of CO₂ formation via carbonates on 5Co/Si C400; both types of carbonates are formed on 5Co/Al C400 to promote the CO oxidation; while both strong adsorption of CO on TiO₂ and CO₂ on cobalt oxide for 5Co/Ti C400 leads to affect the activity.     

Kinetic modeling of CO oxidation on Pt/CeO2 in a gradientless reactor

Cerium oxide is a major additive in three-way catalysts used in emission control of automobile exhaust. Pt/CeO₂ was studied in order to better understand the role of ceria in promoting CO oxidation reaction. The kinetics of carbon monoxide oxidation on Pt/cerium oxide catalyst, was studied over the temperature range 100–170°C. Steady state kinetic measurements of CO oxidation were obtained in a computer controlled micro-CSTR reactor. Activation energies were reported to vary between 39·5 and 51·2 kJ mol⁻¹. At low concentrations of either reactant (CO, O₂) and total conversion, the catalyst exhibited multiple steady states, similar to the multiplicity behavior of Pt/Al₂O₃. The total conversion was reached at 120°C. In comparison, the total conversion at low reactant concentrations was reached at a temperature of 148°C for the alumina-supported catalyst. Langmuir–Hinshelwood mechanisms gave a good fit to the data. However, no single rate expression could effectively describe the CO oxidation data over the whole concentration in the product of the CSTR reactor. The facts gathered indicate that oxygen adsorbed on interfacial Pt/Ce sites and ceria lattice oxygen provides oxygen for CO oxidation. Cerium oxide has been found to lower CO oxidation activation energy, enhance reaction activity and tends to suppress the usual CO inhibition effect.     

Mechanism and kinetics of Al2O3 nanoparticles formation by reaction of bulk Al with H2O and CO2 at sub- and supercritical conditions

Oxidation of bulk samples of 〈Al〉 by water and H₂O/CO₂ mixture at sub- and supercritical conditions for uniform temperature increase and at the injection of H₂O (665 K, 23.1 MPa) and H₂O/CO₂ (723 K, 38.0 MPa) fluids into the reactor has been studied. Transition of 〈Al〉 into AlOOH and Al₂O₃ nanoparticles has been found out. Aluminum samples oxidized by H₂O and H₂O/CO₂ fluids at the injection mostly consist of large particles (300-500 nm) of α-Al₂O₃. Those oxidized for uniform temperature increase contain smaller particles (20-70 nm) of γ-Al₂O₃ as well. Mechanism of this phenomenon is explained by orientation of oxygen in H₂O polar molecules to the metal in the electric field of contact voltage at Al/AlOOH and Al/Al₂O₃ boundary. Addition of CO₂ to water resulted in CO, CH₄, CH₃OH and condensed carbon, increase in size of Al₂O₃ nanoparticles and significant decrease in time delay. In pure CO₂ 〈Al〉 oxidation resulted in oxide film. Using temperature and time dependences of gaseous reactant pressure and Redlich-Kwong state equation, kinetics of H₂ formation has been described and oxidation regularities determined. At aluminum oxidation by H₂O and H₂O/CO₂ fluids, self-heating of the samples followed by oxidation rate increase has been registered. The samples of oxidized aluminum have been studied with a transmission electronic microscope, a thermal analyzer and a device for specific surface measurement. The effect of oxidation conditions on the characteristics of synthesized nanoparticles has been found out.     

Quantitative analyses of oxygen release/storage and CO2 adsorption on ceria and Pt–Rh/ceria

The CO/O₂ and CO₂ pulse experiments were carried out to acquire useful information about oxygen release/storage and CO₂ adsorption on ceria and Pt–Rh/ceria. In the CO pulse experiments at 500 °C, ca. 60% of CO uptake was released as CO₂ while the rest of CO uptake was retained as carbon residuals on the surfaces of both samples. The carbon residuals could be removed when O₂ was provided. In the CO₂ pulse experiments, the adsorption of CO₂ was found to relate to the temperatures and the oxidation states of surface cerium. The reduced Ce³⁺ sites (O vacancies) were responsible for the adsorption of CO₂ at the temperature of 500 °C. In addition, the molar ratios of CO₂ adsorption to O vacancies (38–39%) were in agreement with the ratios of carbon residuals to CO uptake ( ca. 40%) measured in the CO pulse experiments. Quantitative analyses of oxygen release/storage and CO₂ adsorption implied that in the process of oxygen release, carbon residuals were possibly in the form of a carbonate-like species due to the adsorption of CO₂ onto the reduced Ce³⁺ sites.     

Kinetics of the reaction of oxygen with carbon monoxide adsorbed on palladium

A study of the kinetics of the reaction of adsorbed carbon monoxide with oxygen on polycrystalline palladium is reported in which a pressure jump method was used to induce transients in the carbon dioxide production. Through an analysis of these transients under a variety of conditions of temperature and oxygen pressure, some details of the kinetics have been delineated. At relatively low temperatures and under a significant O₂ pressure, CO(a) is desorbed more readily as CO₂, via the reaction CO(a) + O(a) → CO₂, than as CO. The reaction is first order in oxygen and the rate is limited by the rate of adsorption of oxygen onto sites which are in close proximity to CO(a). Oxygen adsorption at sites which are further than a critical distance from CO(a) are unreactive. The critical distance increases with temperature reflecting increased mobility. Under conditions where both CO(a) and O(a) are significant and both CO(g) and O₂(g) are small the rate is limited by the mobility of CO(a) and/or O(a). The amount of CO(a) during the course of the steady-state oxidation reaction can be determined by analyzing the transient CO₂ production which occurs following a pressure jump in carbon monoxide.     

Role of carbon in production of transparent corundum ceramics using the powder alkoxy technology

Poor reproducibility of the transparency of corundum ceramics produced by the alkoxy technology is related to the possibility of penetration of oxidized carbon from alkoxy groups into a solid solutions with Al₂O₃. Carbon neutralizes the effect of magnesium oxide additive and intensifies diffusion mass transfer, capture of pores by growing crystals, and polymorphous transformation into corundum. Introduction of carbon as soot or naphthalene into previously decarbonized Al₂O₃ powder in the course of dry milling, its subsequent heat treatment in air or in moist CO₂ medium, and sintering of the samples in vacuum supported this assumption.     

Ion cyclotron resonance study of CO oxidation in the gas phase in the presence of rhenium cations with carbonyl and oxygen ligands. Comparison with heterogeneous catalysis

Gas-phase oxidation of CO in the presence of rhenium cations with carbonyl and oxygen ligands has been studied by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. Rhenium cations have been generated by the electron impact of Re₂(CO)₁₀ vapour. Contrary to the unreactive rhenium ions, rhenium monocarbonyi ions have been found to react with O₂ molecules yielding rhenium monoxide ions and CO₂ molecules. ReO⁺ ions are subsequently oxidized with O₂ to di- and trioxide ions. The bond energies in rhenium oxide ions were estimated as D°(Re⁺–O)=104±14, D°(ReO⁺–O)<118, D°ReO ₂ ⁺ –O)=122±4 kcal/mol. Simultaneous addition of CO and O₂ molecules to the reaction volume leads to the gas-phase catalytic oxidation of CO with pairs of rhenium oxide ions ReO ₃ ⁺ /ReO ₂ ⁺ serving as the oxidized and reduced forms of the catalyst. The mechanisms of the above reactions are discussed in connection with that for oxidation of CO over solid oxide catalysts.     

Catalytic steam reforming of ethane and propane over CeO2-doped Ni/Al2O3 at SOFC temperature: Improvement of resistance toward carbon formation by the redox property of doping CeO2

Ni/Al₂O₃ with the doping of CeO₂ was found to have useful activity to reform ethane and propane with steam under Solid Oxide Fuel Cells (SOFCs) conditions, 700-900 °C. CeO₂-doped Ni/Al₂O₃ with 14% ceria doping content showed the best reforming activity among those with the ceria content between 0 and 20%. The amount of carbon formation decreased with increasing Ce content. However, Ni was easily oxidized when more than 16% of ceria was doped. Compared to conventional Ni/Al₂O₃, 14%CeO₂-doped Ni/Al₂O₃ provides significantly higher reforming reactivity and resistance toward carbon deposition. These enhancements are mainly due to the influence of the redox properties of doped ceria. Regarding the temperature programmed reduction experiments (TPR-1), the redox properties and the oxygen storage capacity (OSC) for the catalysts increased with increasing Ce doping content. In addition, it was also proven in the present work that the redox of these catalysts are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results.During the reforming process, in addition to the reactions on Ni surface, the gas-solid reactions between the gaseous components presented in the system (C₂H₆, C₃H₈, C₂H₄, CH₄, CO₂, CO, H₂O, and H₂) and the lattice oxygen (O_x) on ceria surface also take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen (O_x) on ceria surface (C_nH_m+O_x→nCO+m/2(H₂)+O_x−n) can prevent the formation of carbon species on Ni surface from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). Moreover, the formation of carbon via Boudard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide (produced from steam reforming) with the lattice oxygen (CO+O_x⇔CO₂+O_x−1).     

Performance of CuO/Al2O3 Catalysts Promoted by SnO2 and ZrO2 in the Selective Oxidation of Carbon Monoxide

The effect of added SnO₂ and ZrO₂ to CuO/Al₂O₃ catalysts was investigated with reference to the oxygen spillover phenomena in the selective oxidation of carbon monoxide. The TPR and TPO analyses indicated that SnO₂ and ZrO₂ addition caused oxygen migration and induced the formation of high concentrations of active oxygen species on the SnO₂ and ZrO₂ surface. The catalytic activities of SnO₂ and ZrO₂ supported CuO/Al₂O₃ catalysts were superior to that for CuO/Al₂O₃ catalysts in the selective oxidation of carbon monoxide. Oxygen, when absorbed to the SnO₂ and ZrO₂ surface can spill over to the CuO phase and easily react with carbon monoxide. Consequently, the addition of SnO₂ and ZrO₂ led to significantly improved activities. This can be attributed to the enhanced migration of oxygen to the catalyst surface.     

Non-Faradaic catalysis: the case of CO oxidation over Ag-Pd alloy electrode in a solid oxide electrolyte cell

Studies of carbon monoxide oxidation on an Ag-25 at% Pd alloy electrode in a cell with a solid oxygen-conducting electrolyte - CO+O₂, Ag-Pd | 0.9 ZrO₂+0.1Y₂O₃ | Pt+PrO_2-x, air - were carried out. XRD, SEM and XPS techniques were used for characterisation of the Ag-Pd alloy electrode. The non-Faradaic effect of electrochemical oxygen pumping on the rate of carbon monoxide oxidation was demonstrated. The induced change in the reaction rate at cathodic polarization of the Ag-Pd alloy electrode was an order of magnitude higher than the rate of oxygen pumping from the reaction zone through the electrolyte. The observed phenomenon was qualitatively explained on the base of a chain reaction mechanism involving electrochemically generated oxygen species.     

Characterization of carbon deposits on alumina supported cobalt and nickel by temperature programmed gasification with O2, CO2 and H2

Temperature-programmed gasification with O₂, CO₂ and H₂ is used for the characterization of carbon deposits on Ni/Al₂O₃ and Co/Al₂O₃, based on the difference in reactivity of the several types of deposits. These deposits were formed by CO disproportionation at 675 K, 725 K and 775 K. The gasification patterns show the existence of mainly two types of carbon deposits. One form is attributed to filamentous carbon, the second form is believed to have a more graphitic structure. The TPG patterns are strongly influenced by the oxidation state and the catalytic activity of the metal(oxide) on the gasification of carbon.     

Gas Analysis of Carbon Oxidation in a Coin‐Type Direct Carbon Fuel Cell

Carbon oxidation behaviors were illuminated in terms of gas composition in a coin‐type direct carbon fuel cell. The main gas species in the anode chamber at 850 °C was mostly carbon monoxide, which was generated from the chemical reaction of carbon and molten carbonates. The concentration of CO was reduced as time passed because the reactivity of carbonates was weakened. The open circuit voltage was directly dependent on the CO concentration. The gases in the anode chamber had a vertical concentration distribution; the highest CO and the lowest CO₂ concentrations were observed near the electrode. However, the voltage in the polarization state was less dependent on the gas composition. A polarization state of 150 mA cm^–2 allowed the oxidation of CO, resulting in an increased CO₂ concentration near the electrode. The enlarged CO₂ partial pressure facilitated CO generation through the recombination of carbonate ions (CO₃^2–). Decreasing the temperature from 850 to 750 °C reduced the level of carbon monoxide at the anode. The presence of CO as a main component in the anode concludes that the oxidation of solid carbon takes place through the gasification of carbon to CO, then electrochemically to CO₂.     

FT-IR study of Au/Fe2O3 catalysts for CO oxidation at low temperature

Coprecipitated Au/Fe₂O₃ catalysts used for low-temperature catalytic oxidation of carbon monoxide have been studied by FT-IR spectroscopy of adsorbed CO. The FT-IR results have shown that after preparation and exposure to a CO/O₂ mixture gold is present on the surface mainly as Au¹⁺ and Au⁰ species. It has been found that in the CO oxidation Au¹⁺ is more active and less stable than Au⁰. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen production through dry reforming of biogas using a porous electrochemical cell: Effects of a cobalt catalyst in the electrode and mixing of air with biogas

This paper reports the performance of porous Gd-doped ceria (GDC) electrochemical cells with Co metal in both electrodes (cell No. 1) and with Ni metal in the cathode and Co metal in the anode (cell No. 2) for CO₂ decomposition, CH₄ decomposition, and the dry reforming reaction of a biogas with CO₂ gas (CH₄ + CO₂ → 2H₂ + 2CO) or with O₂ gas in air (3CH₄ +?1.875CO₂ +?1.314O₂ → 6H₂ +?4.875CO +?0.7515O₂). GDC cell No. 1 produced H₂ gas at formation rates of 0.055 and 0.33?mL-H₂/(min?m²-electrode) per 1?mL-supplied gas/(min?m²-electrode) at 600?°C and 800?°C, respectively, by the reforming of the biogas with CO₂ gas. Similarly, cell No. 2 produced H₂ gas at formation rates of 0.40?mL-H₂/(min?m²) per 1?mL-supplied gas/(min?m²) at 800?°C from a mixture of biogas and CO₂ gas. The dry reforming of a real biogas with CO₂ or O₂ gas at 800?°C proceeded thermodynamically over the Co or Ni metal catalyst in the cathode of the porous GDC cell. Faraday's law controlled the dry reforming rate of the biogas at 600?°C in cell No. 2. This paper also clarifies the influence of carbon deposition, which originates from CH₄ pyrolysis (CH₄ → C + 2H₂) and disproportionation of CO gas (2CO → C + CO₂), on the cell performance during dry reforming. The dry reforming of a biogas with O₂ molecules from air exhibits high durability because of the oxidation of the deposited carbon by supplied air.     

Influence of preparation method on performance of Cu(Zn)(Zr)-alumina catalysts for the hydrogen production via steam reforming of methanol

The selective production of hydrogen via steam reforming of methanol (SRM) was performed using prepared catalysts at atmospheric pressure over a temperature range 200–260^∘C. Reverse water gas shift reaction and methanol decomposition reactions also take place simultaneously with the steam reforming reaction producing carbon monoxide which is highly poisonous to the platinum anode of PEM fuel cell, therefore the detailed study of effect of catalyst preparation method and of different promoters on SRM has been carried out for the minimization of carbon monoxide formation and maximization of hydrogen production. Wet impregnation and co-precipitation methods have been comparatively examined for the preparation of precursors to Cu(Zn)(Al₂O₃) and Cu(Zn)(Zr)(Al₂O₃). The catalyst preparation method affected the methanol conversion, hydrogen yield and carbon monoxide formation significantly. Incorporation of zirconia in Cu(Zn)(Al₂O₃) catalyst enhanced the catalytic activity, hydrogen selectivity and also lower the CO formation. Catalyst Cu(Zn)(Zr)(Al₂O₃) with composition Cu/Zn/Zr/Al:12/4/4/80 prepared by co-precipitation method was the most active catalyst giving methanol conversion up to 97% and CO concentration up to 400 ppm. Catalysts were characterized by atomic absorption spectroscopy (AAS), Brunauer-Emett-Teller (BET) surface area, pore volume, pore size and X-ray powder diffraction (XRPD). The XRPD patterns revealed that the addition of zirconia improves the dispersion of copper which resulted in the better catalytic performance of Cu(Zn)(Zr)(Al₂O₃). The time-on-stream (TOS) catalysts stability test was also conducted for which the Cu(Zn)(Zr)(Al₂O₃) catalyst gave the consistent performance for a long time compared to other catalysts.     

Influence of preparation method on performance of Cu(Zn)(Zr)-alumina catalysts for the hydrogen production via steam reforming of methanol

The selective production of hydrogen via steam reforming of methanol (SRM) was performed using prepared catalysts at atmospheric pressure over a temperature range 200–260^°C. Reverse water gas shift reaction and methanol decomposition reactions also take place simultaneously with the steam reforming reaction producing carbon monoxide which is highly poisonous to the platinum anode of PEM fuel cell, therefore the detailed study of effect of catalyst preparation method and of different promoters on SRM has been carried out for the minimization of carbon monoxide formation and maximization of hydrogen production. Wet impregnation and co-precipitation methods have been comparatively examined for the preparation of precursors to Cu(Zn)(Al₂O₃) and Cu(Zn)(Zr)(Al₂O₃). The catalyst preparation method affected the methanol conversion, hydrogen yield and carbon monoxide formation significantly. Incorporation of zirconia in Cu(Zn)(Al₂O₃) catalyst enhanced the catalytic activity, hydrogen selectivity and also lower the CO formation. Catalyst Cu(Zn)(Zr)(Al₂O₃) with composition Cu/Zn/Zr/Al:12/4/4/80 prepared by co-precipitation method was the most active catalyst giving methanol conversion up to 97% and CO concentration up to 400 ppm. Catalysts were characterized by atomic absorption spectroscopy (AAS), Brunauer-Emett-Teller (BET) surface area, pore volume, pore size and X-ray powder diffraction (XRPD). The XRPD patterns revealed that the addition of zirconia improves the dispersion of copper which resulted in the better catalytic performance of Cu(Zn)(Zr)(Al₂O₃). The time-on-stream (TOS) catalysts stability test was also conducted for which the Cu(Zn)(Zr)(Al₂O₃) catalyst gave the consistent performance for a long time compared to other catalysts.     

Direct oxidation of dry methane on nanocrystalline Ce0.8Gd0.2O2-δ/Pt anodes

The electrocatalytic activity of composite anodes, consisting of micron-scale sized Pt particles and nanocrystalline Ce_0.8Gd_0.2O_2-δ(CGO) prepared by the cellulose-precursor technique, was evaluated for the oxidation of dry methane in a solid oxide fuel cell (SOFC) with zirconia-based electrolyte at 1173 K. Increasing current density above 100 mA/cm² and the corresponding decrease of CH₄/O₂ ratio down to 2–3 suppressed carbon formation, but decreased CO/CO₂ molar ratio in the product mixture to 0.3–0.9. The methane conversion rate was found to increase linearly with current, suggesting an increasing role of total CH₄ oxidation by oxygen electrochemically supplied onto the anode surface. The results show that, although ceria-based anode components are well known to improve SOFC performance, their presence leads to high CO₂ selectivity and thus seems inappropriate for the generation of synthesis gas in SOFC-type reactors.     

Study of the NOχ reaction with reducing gases on Fe/ZrO2 catalyst

The Fe/ZrO₂ catalyst (1% Fe by weight) shows a strong adsorption capacity toward the nitric oxide (at room temperature the ratio NOFe is ca. 0.5) as a consequence of the formation of a highly dispersed iron phase after reduction at 500–773 K. Nitric oxide is adsorbed mainly as nitrosyl species on the reduced surface where the Fe²⁺ sites are prevailing, but it is easily oxidised by oxygen forming nitrito and nitrato species adsorbed on the support. However, in the presence of a reducing gas such as hydrogen, carbon monoxide, propane and ammonia at 473–573 K the Fe-nitrosyl species react producing nitrogen, nitrous oxide, carbon dioxide and water, as detected by FTIR and mass spectrometers. The results show that nitric oxide reduction is more facile with hydrogen containing molecules than with CO, probably due the co-operation of spillover effects. Experiments carried out with the same gases in the presence of oxygen show, however, a reduced dissociative activity of the surface iron sites toward the species NO_χ formed by NO oxidation and therefore the reactivity is shifted to higher temperatures.     

2D-HARECXS analysis of dopant and oxygen vacancy sites in Al-doped yttrium titanate

A Y₂Ti₂O₇/Al₂O₃ multilayer, exhibiting high thermal reflectivity and oxidation resistivity, holds potential as a functional surface coating for advanced gas turbine blades. Slight Al doping of Y₂Ti₂O₇ has been found to significantly improve the structural stability of the multilayer, the detailed mechanism of which remains unclear. Hence, we examined the site occupancies of doped Al using high-angular resolution electron channeling X-ray spectroscopy with two-dimensional rocking of the incident electron beam. A statistical linear regression analysis of a set of observed ionization channeling patterns (ICPs) of fluorescent X-rays quantitatively confirmed that Al³⁺ preferentially occupied the Ti⁴⁺ site, rather than the Y³⁺ site, because of the large difference in the ionic radii of Al³⁺ and Y³⁺. Furthermore, the comparison of theoretical and experimental X-ray ICPs showed that oxygen vacancies V_O were introduced at the 48f site, the first-nearest neighbor of the Ti site, which was consistent with the hypothesis that oxygen vacancies compensate for the local charge imbalance associated with preferential substitution of Al³⁺ for Ti⁴⁺. Our findings can aid in improving the thermal properties of environmental barrier coatings applied to advanced jet engines and also demonstrate the utility of beam-rocking schemes for investigating dopant effects in various functional materials.