CO Oxidation Behavior Research Articles

Low temperature CO oxidation by doped cerium oxide electrospun fibers

We investigated CO oxidation behavior of doped cerium oxide fibers. Electrospinning technique was used to fabricate the inorganic fibers after burning off polymer component at 600 °C in air. Cu, Ni, Co, Mn, Fe, and La were doped at 10 and 30 mol% by dissolving metal salts into the polymeric electrospinning solution. 10 mol% Cu-doped ceria fiber showed excellent catalytic activity for low temperature CO oxidation with 50% CO conversion at just 52 °C. This 10 mol% Cu-doped sample showed unexpected regeneration behavior under simple ambient air annealing at 400 °C. From the CO oxidation behavior of the 12 samples, we conclude that absolute oxygen vacancy concentration estimated by Raman spectroscopy is not a good indicator for low temperature CO oxidation catalysts unless extra care is taken such that the Raman signal reflects oxide surface status. The experimental trend over the six dopants showed limited agreement with theoretically calculated oxygen vacancy formation energy in the literature.

Morphology and size of Pt on Al2O3: The role of specific metal-support interactions between Pt and Al2O3

The catalytic performance of supported heterogeneous catalysts is significantly affected by the particle size and morphologies of the active phase. In this work, we studied how metal-support interaction affected the morphology and size of Pt clusters on Al2O3. Under oxidizing atmosphere, when the specific interaction between oxidized Pt and Al2O3 is dominant, highly dispersed 2D PtO2 rafts (1 nm) and atomically dispersed Pt atoms formed. At high Pt loadings, when only a fraction of the metal atoms can interact with the alumina support, or on weakly interacting supports (e.g., SiO2), 3D PtO2 particles (>2 nm) and large Pt crystallites (>10 nm) formed. Due to the weak interaction with the support, these 3D PtO2 particles can be reduced at a lower temperature (−40 °C) than 2D PtO2 rafts (~110 °C) and atomically dispersed Pt (>300 °C). The catalytic behavior for CO oxidation is also affected by the reduction properties of supported Pt catalysts. The activity of Pt/Al2O3 stepwise increases with the reduction temperature, which is closely related with the reduction of these three types of oxidized Pt species. In contrast, Pt/SiO2 didn’t show drastic activity change with the subsequent reduction treatments up to 500 °C because it contains mainly 3D PtO2 particles. All these results demonstrate that specific metal-support interactions guide the geometric and chemical properties and the corresponding catalytic performances of oxidized Pt in Pt/Al2O3 catalysts, underlining the importance of careful catalyst activation for the efficient utilization of metallic Pt in real-world catalytic applications.

Effect of Ti addition on high-temperature oxidation behavior of Co–Ni-based superalloy

The oxidation behavior of a newly designed Co–Ni-based alloy with varied addition of Ti (1.4 and 2.1 wt.%, hereafter referred as 1.4Ti and 2.1Ti alloys) was explored in air at 900 °C. It is found that an outer oxide layer composed of CoO, an intermediate oxide layer composed of intermittent TiO2 plus continuous Cr, W-rich oxides, and an inner oxidized region composed of Al2O3 were formed on the surface of 1.4Ti alloy, whereas the oxide structure of 2.1Ti alloy was composed of CoO as the outer oxide layer, continuous TiO2, Cr2O3, and WO3 as intermediate oxide layer, and Al2O3 as the inner oxidized region. With increasing Ti content, continuous TiO2 films formed in the intermediate oxide layer, which would lead to the formation of compact Cr2O3 and improved oxidation resistance after 100 h oxidation at 900 °C. These observations indicated that Ti addition can improve the high-temperature oxidation resistance of Co–Ni-based alloys.

Influence of W Content on the Oxidation Behaviour of Ternary $$\gamma ^{\prime }$$-Strengthened Co-Based Model Alloys Between 800 and $$900\,^{\circ }{\hbox {C}}$$

The oxidation behaviour of ternary Co–Al–W alloys with W contents between 7 and 10 at. $$\%$$ was investigated in detail. Continuous thermogravimetry was conducted between 800 and $$900\,^{\circ }{\hbox {C}}$$ to compare the impact of $$\gamma ^{\prime }$$ volume fraction on the overall mass gain during the comparably long transient stages of oxide-scale growth. Resulting cross-sections were assessed using electron microscope-based analysis methods. To understand prevailing oxidation mechanisms, the development and composition of multilayered oxide scales were the main focus of the present study. Elemental distribution of three individual scale sections was investigated with electron probe microanalysis. No striking impact of the W level in the alloy was found. However, the development of diffusion-limiting barrier layers was demonstrated to change with varying W contents. Continuous alumina layers were only observed after 100-h exposure at $$800\,^{\circ }{\hbox {C}}$$ . For samples with 9 at. $$\%$$ W, the grain structure within the outer Co-oxide layer was characterized with electron backscattered diffraction after the maximum duration of exposure. Individual growth kinetics for three distinguished layers within the oxide scales were used to compare elemental transport depending on the W content in the alloy and the exposure temperature.

Supercritical water oxidation of ammonia with methanol as the auxiliary fuel: Comparing with isopropanol

The co-oxidation behavior of methanol on ammonia in supercritical water was investigated in a continuous tubular reactor under a range of methanol concentration (10–800 mmol/L), ammonia concentration (50–400 mmol/L), oxidation coefficient (1.05–1.7) and preheating temperature of 400 and 415 °C. Calculation based on a mechanism-based model for supercritical water oxidation of methanol/ammonia showed that the synergy between the kinetics action to supply radicals and the thermal action to elevate the reaction temperature of methanol would improve the efficient decomposition of ammonia at relatively moderate reaction conditions. Specifically, near complete conversion of ammonia (97.5%) can be achieved at [CH3OH]0 = 800 mmol/L, preheating temperature of 415 °C and oxidation coefficient of 1.5. The influence of reaction conditions on the co-oxidation behavior and the formation of nitrate were examined experimentally. Methanol and isopropanol served as the auxiliary fuel for ammonia were compared by the positive effect and exothermic performance, which reached that methanol was superior to isopropanol.

Does hydrothermal carbonization as a biomass pretreatment reduce fuel segregation of coal-biomass blends during oxidation?

Co-firing of biomass and coal may increase short-term renewable fuel usage. However, the lower heating value and higher reactivity (at lower temperatures) of raw biomass can result in fuel segregation in boilers, causing burnout at lower temperatures, lower steam generation efficiency and fouling. In addition, the relatively high water content of some biomasses requires extensive drying prior to combustion. These issues may be addressed by hydrothermally carbonizing moist biomasses to produce hydrochars that more closely resemble coal’s properties prior to co-firing. In the present work, we probe the co-oxidation behavior of a series of hydrothermally carbonized biomass samples over a range of hydrochar-coal blend ratios to determine the degree of carbonization necessary to reduce fuel segregation. However, due to the presence of an extractable amorphous secondary char, even highly carbonized hydrochars have considerably higher oxidative reactivity than a representative bituminous coal sample. When blended, the hydrochars and coal display distinct derivative thermogravimetric oxidation ranges, in which a low-temperature peak is dominated by the secondary char oxidation, followed by a high-temperature char oxidation peak of the primary solid hydrochar. After extraction of the secondary char, the primary char displays a lower reactivity than the coal. As a co-fired fuel, it appears that blending hydrochars up to 10 wt% with a bituminous coal is possible as a partial fuel substitution. To increase the percentage of hydrochars blended with coal, it may be necessary to extract this secondary char (which contains valuable biofuels and platform chemicals) before blending.

Thermogravimetric investigations of new \u03b3-\u03b3\u2032 cobalt-based superalloys

The aim of the study is characterization of high-temperature oxidation behavior of new Co-based superalloys by thermogravimetry (TG). The following alloys have been taken into account: Co–9Al–9W, Co–20–Ni–7Al–7W, Co–10Al–5Mo–2Nb and Co–20Ni–10Al–5Mo–2Nb (at.%). The thermogravimetric analysis was carried out in the temperature range 40–1200 °C of heating rate 5° min−1. The investigation was performed using the thermal analyzer NETZSCH STA 449 F3 Jupiter. TG–DTG plots showed oxidation behavior up to 1200 °C and indicated the temperatures of further isothermal oxidation examinations. The oxidation behavior of basic Co–9Al–9W (at.%) alloy was compared to W-free Co–10Al–5Mo–2Nb and Co–20Ni–10Al–5Mo–2Nb (at.%) alloys. The obtained data showed different oxidation behavior dependably on the type of alloying elements. Moreover, the effect of Ni addition on oxidation performance was determined. The scales grown on Co-base superalloys after thermogravimetry were evaluated by means of scanning electron microscopy and energy dispersive spectroscopy.

O 2 activation and CO oxidation on n-p codoped h-BN single-atom catalysts

Abstract The O2 activation and CO oxidation behaviors on the n-p codoped hexagonal boron nitride (h-BN) monolayer have been systematically investigated by first-principles calculation. It firstly shows that the stable single-atom catalysts can be formed with the help of stronger electrostatic attraction from the n-type metals (Fe, Cu, Pd, Ag, Ir, Pt, and Au) and p-type C doped h-BN monolayer. Secondly, the obviously elongated O O bond of O2 suggests that it has been well activated on n-p codoped h-BN monolayer. As a prototype example, we check CO oxidation reaction on C/Cu codoped h-BN monolayer and find that the maximum energy barrier of CO oxidation is 0.655 eV by the Eley-Rideal mechanism. Furthermore, the strongly hybridization between Cu 3d and O2 2p orbitals is found to plays a crucial role in the CO oxidation on n-p codoped h-BN monolayer.

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co3O4, Co(Ni)O and Ni-doped CoCr2O4, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr2O3 layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.

Co-oxidation of CO and propylene on Pd/CeO2-ZrO2 and Pd/Al2O3 monolith catalysts: A light-off, kinetics, and mechanistic study

The light-off (ignition) and steady-state behavior for individual oxidation and co-oxidation of CO and propylene under near-stoichiometric conditions was studied using Pd/Al2O3 and Pd/CeO2-ZrO2 monolith catalysts. CO and propylene are shown to be self- and mutually-inhibiting, with inhibition mitigated by the promotional effect of CeO2-ZrO2. Oxidation is enhanced at low temperatures for CO, and at intermediate and high temperatures for propylene. Light-off behavior during CO and propylene co-oxidation is similarly improved at low and high temperatures. Steady-state differential kinetics measurements using Pd/Al2O3 show reaction orders of ∼−1 with respect to CO and propylene. Using Pd/CeO2-ZrO2, the reaction order with respect to CO shifts towards zero and the activation energy decreases, suggesting an alternate reaction mechanism for CO oxidation when enough ceria is present. Mechanisms for CO and propylene oxidation that are consistent with the kinetics and inhibition trends are presented.

Near Ambient Pressure XPS Investigation of CO Oxidation Over Pd3Au(100)

The CO oxidation behavior under excess oxygen and near stoichiometric conditions over the surface of Pd3Au(100) has been studied by combining near-ambient pressure X-ray photoelectron spectroscopy and quadrupole mass spectrometry and compared to Pd(100). During heating and cooling cycles, normal hysteresis in the CO2 production, i.e. with the light-off temperature being higher than the extinction temperature, is observed for both surfaces. On both Pd3Au(100) and Pd(100) the (√5 × √5)R27° surface oxide structure is present during CO2 production under excess oxygen conditions (O2:CO = 10:1), while at near stoichiometric conditions (O2:CO = 1:1) the surfaces are covered with atomic oxygen. Au as alloying element hence induces only minor differences in the observed hysteresis and the active phase compared to pure Pd. Alloying with Au thus yields a different behavior compared to Ag, where reversed hysteresis is observed for CO2 production over Pd75Ag25(100) at similar conditions [Fernandes et al., ACS Catal. (2016) 4154].

Catalytic activity of infiltrated La0.3Sr0.7Ti0.3Fe0.7O3−δ–CeO2 as a composite SOFC anode material for H2 and CO oxidation

Finding cost-effective and efficient anode materials for solid oxide fuel cells (SOFCs) is of prime importance to develop renewable energy technologies. In this paper, La and Fe co-doped SrTiO3 perovskite oxide, La0.3Sr0.7Ti0.3Fe0.7O3−δ (LSTF0.7) composited with CeO2 is prepared as a composite anode by solution infiltration method. The H2 and CO oxidation behavior and the electrochemical performance (electrochemical impedance spectra, I–V and I–P curves) of the scandia-stabilized zirconia (ScSZ) electrolyte supported cells fabricated by tape casting with the LSTF0.7–CeO2 composite anode are subsequently measured at various temperatures (700–850 °C). Electrochemical impedance spectra (EIS) of the prepared cells with the LSTF0.7–CeO2|ScSZ|La0.8Sr0.2MnO3 (LSM)–ScSZ configuration illustrate that the anode polarization resistance distinguished from the whole cell is 0.072 Ω cm2 in H2, whereas 0.151 Ω cm2 in CO at 850 °C. The maximal power densities (MPDs) of the cell at 700, 750, 800 and 850 °C are 217, 462, 612, 815 mW cm−2 in H2 and 145, 349, 508, 721 mW cm−2 in CO, respectively. Moreover, a significant decrease of anode activation energy towards H2 oxidation is clearly demonstrated, indicating a better electrochemical performance in H2 than in CO. These results demonstrate an alternative composite anode with high electrocatalytic activity for SOFC practical applications.

Biotemplate Synthesis of Micron Braid Structure CeO2-TiO2 Composite and Analysis of its Catalytic Behavior for CO Oxidation

A series of CeO₂-TiO₂ composite samples with different Ce/Ti molar ratios were prepared by the paper template. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to confirm a face-centered cubic lattice of CeO₂ with Ce/Ti =8:2 or 9:1 and a two phase mixture of anatase titania and face-centered cubic ceria with Ce/Ti = 7 : 3. The field emission scanning electron microscopy (FESEM) results suggest that the products are micron braid structures consisting of fibers with diameters in a range of 1-6 ㎛ and lengths of several hundred micrometers. N₂ absorption-desorption testing shows that the composite at Ce/Ti molar fraction of 8 : 2 has the largest BET surface area (about 81 ㎡·g −1 ). Compared to the pure CeO₂ sample, the composites show superior catalytic activity for H₂ reduction and CO oxidation. For the micron braid structure CeO₂-TiO₂ composite (Ce/Ti = 8 : 2), due to the high surface area and the solid solution with appropriate Ti4+ incorporation, the CO conversion at about 280℃ was above 50% and at 400℃ was 100%.

Benchmarking of manganese oxide materials with CO oxidation as catalysts for low temperature selective oxidation

CO oxidation behavior of different manganese oxide materials was studied as a model probe reaction to benchmark catalytic activities that could be useful for other aerobic oxidations. K-OMS-2, which showed intermediate activity [Ea=3.0 and 6.3kcal/mol for K-OMS-2 (made in solvent free procedure or SF) and K-OMS-2 (made via hydrothermal route or REF) respectively], were chosen for metal dopant studies using microwave assisted hydrothermal synthesis. These were further characterized using X-ray diffraction (XRD), N2 sorption, Raman spectroscopy, scanning electron microscopy (SEM), and carbon monoxide temperature programmed reduction (CO-TPR). Additional characterization was carried out for Ni-K-OMS-2 materials using atomic absorption spectroscopy (AAS) and thermogravimetric analysis-mass spectroscopy (TGA-MS). Doping K-OMS-2 with Ni resulted in a large increase in oxygen mobility and re-adsorption, and the moderate activation energy for CO oxidation (13.kcal/mol) by Ni-K-OMS-2(SF) suggests that Ni-doped materials are good candidates for low temperature selective oxidation.

Room Temperature CO Oxidation over Pt/MgFe2O4: A Stable Inverse Spinel Oxide Support for Preparing Highly Efficient Pt Catalyst.

MgFe2O4 with inverse spinel structure is demonstrated to be an efficient support for constructing practical potential Pt catalyst (Pt/MgFe2O4). The resultant Pt/MgFe2O4 exhibits excellent catalytic behavior in CO oxidation under normal temperature and humidity. TOF calculated based on the content of Pt is 0.131 s-1. The excellent performance of Pt/MgFe2O4 attributes to the presence of surface undercoordinated lattice oxygens on MgFe2O4 support. These oxygens could participate in the initial CO oxidation and then be recovered under O2 conditions. Over this Pt/MgFe2O4 catalyst, CO catalytic oxidation should mainly follow a redox mechanism.

High temperature oxidation behavior of laser clad Co–Cr–Mo coating on γ-TiAl substrate

In this work, Co–Cr–Mo powder is used to form laser clads on a γ-TiAl substrate. The oxidation behavior of Co-Cr-Mo alloys in air at 900 °C for 10 and 480 h has been studied. The results show that the uniform, fine, and defect-free microstructure of Co-Cr-Mo coating was prepared by laser cladding. The clad coating consisted of γ-Co (fcc), ε-Co (hcp), and Cr23C6. The specific mass gain as a function with the number of cycle plot was parabolic, which indicated that diffusion has controlled the oxidation process. At short time (10 h), the oxide scale formed on the surface was composed mainly of Cr2O3. With the increase in the duration of oxidation from 10 to 480 h, the thickness of the oxide film increased from about 1–1.5 to 6–6.5 μm. The analyses of oxide scale formed after long time (480 h) showed that layer II composed of CoCr2O4 formed on the Cr2O3 (layer I). When the thickness of the Cr2O3 layer increased, the defects such as porosity and microchannels were formed in oxide scale. Cobalt diffused from these defects to surface and with increasing the oxidation time, CoCr2O4 formed and covered the surface of oxide scale.

Understanding the Bifunctional Effect for Removal of CO Poisoning: Blend of a Platinum Nanocatalyst and Hydrous Ruthenium Oxide as a Model System

CO poisoning of Pt catalysts is one of the most critical problems that deteriorate the electrocatalytic oxidation and reduction reactions taking place in fuel cells. In general, enhancing CO oxidation properties of catalysts by tailoring the electronic structure of Pt (electronic effect) or increasing the amount of supplied oxygen species (bifunctional effect), which is the typical reactant for CO oxidation, has been performed to remove CO from the Pt surface. However, though there have been a few reports about the understanding of the electronic effect for rapid CO oxidation, a separate understanding of bifunctional modification is yet to be achieved. Herein, we report experimental investigations of CO oxidation in the absence of electronic effect and an extended concept of the bifunctional effect. A model system was prepared by blending conventional Pt/C catalysts with hydrous ruthenium oxide particles, and the CO oxidation behaviors were investigated by various electrochemical measurements, including C...

Composition and catalytic behavior in CO oxidation of biogenic iron-containing materials

The composition of biomasses obtained by the cultivation of Leptothrix genus bacteria in Adler, Fedorov, and Lieske nutrition media has been studied by infrared and Mossbauer spectroscopy. The catalytic activity of the biogenic materials in the reaction of CO oxidation was tested using in situ diffuse-reflectance infrared spectroscopy. The fresh materials contained α-FeOOH, γ-FeOOH, and γ-Fe2O3 at different ratios, one component being predominant. A noticeable catalytic activity of all samples was registered at 250 °C. The catalytic performance depended on FeOOH → Fe2O3 transitions. Spent samples contained mixtures of α-FeOOH and γ-FeOOH as well as α-Fe2O3 and γ-Fe2O3. The catalytic activity up to 200 °C was assigned to the oxyhydroxides, whereas that at 250 °C is a result of α-Fe2O3 and γ-Fe2O3 formation during the process. The oxyhydroxides showed low catalytic activity in the applied reaction, maghemite exhibited a moderate catalytic activity, whereas hematite activity was commensurate with that of the oxyhydroxides. The catalytic activity in CO oxidation was inherent for biogenic maghemite.

Mild activation of CeO2-supported gold nanoclusters and insight into the catalytic behavior in CO oxidation.

We report a new activation method and insight into the catalytic behavior of a CeO2-supported, atomically precise Au144(SR)60 nanocluster catalyst (where thiolate -SR = -SCH2CH2Ph) for CO oxidation. An important finding is that the activation of the catalyst is closely related to the production of active oxygen species on CeO2, rather than ligand removal of the Au144(SR)60 clusters. A mild O2 pretreatment (at 80 °C) can activate the catalyst, and the addition of reductive gases (CO or H2) can enhance the activation effects of O2 pretreatment via a redox cycle in which CO could reduce the surface of CeO2 to produce oxygen vacancies-which then adsorb and activate O2 to produce more active oxygen species. The CO/O2 pulse experiments confirm that CO is adsorbed on the cluster catalyst even with ligands on, and active oxygen species present on the surface of the pretreated catalyst reacts with CO pulses to generate CO2. The Au144(SR)60/CeO2 exhibits high CO oxidation activity at 80 °C without the removal of thiolate ligands. The surface lattice-oxygen of the support CeO2 possibly participates in the oxidation of CO over the Au144(SR)60/CeO2 catalyst.

CO adsorption on TiO2 nanomaterials: nanoparticles, nanotubes and nano-surfaces

The adsorption and oxidation behaviour of CO on different structural TiO2 has been analysed based on the density function theory first principle calculation. For zero-dimensional structure of nanoparticles, a high-specific surface area and onefold oxygen atoms on the surface result in a high rate of adsorption and oxidation for gas molecules. However, for one-dimensional nanotubes, CO molecules are physisorbed because of the absence of coordinatively unsaturated Ti4+ or basic O2− sites on it. But with advantages of lower levels of recombination of charge carriers, chemical adsorption may happen after illumination by the presence of adsorbate states in the band gap of the TiO2 nanotube upon light adsorption. Two-dimensional surfaces with onefold oxygen atoms or hole-assisted surface planes have stronger adsorption and oxidation ability.