Co-oxidation Of CO Research Articles

Effect of dopants on soot oxidation over doped Ag/ZrO2 catalysts for catalyzed gasoline particulate filter

Soot oxidation was studied using silver-zirconia catalysts doped with Y, La, Ce and Pr under a low concentration of O2, typical for gasoline direct injection engine conditions, in view of practical gasoline particulate filter application. The Ag/La-ZrO2 catalyst revealed the best catalyst performance for soot oxidation exhibiting a Tmax of 405 °C for a fresh sample and 420 °C after aging at 700 °C for 6 h. Additionally, the doped catalysts exhibited improved activity for co-oxidation of CO and C3H6. The Ag/ZrO2 catalyst benefited from doping via the synergy between silver particles and doped zirconia.

Theoretical analysis of mixed open-circuit potential for high temperature electrochemical cells electrodes

The Nernst equilibrium potential calculates the theoretical OCV, which estimates the best performance achievable by an electrochemical cell. When multiple semi-reactions (or multiple ionic species) are active in one of the electrodes, the calculation of the theoretical OCV is not straightforward, since different Nernst potentials are associated to each semi-reaction. In this paper, analytical equations for calculation of the theoretical OCV are developed, using the mixed potential theory. The case of H2 and CO co-oxidation (or H2O and CO2 co-reduction) in solid oxide cells is used as a reference case, but similar conclusions can be drawn for other equivalent cases. OCV data from literature are used to calibrate and validate the model. The relative reaction rate of H2 and CO semi-reactions is estimated within the calibration process, and the result is in line with assumptions and suggestions given by other authors. The validation procedure shows predicted OCV values in line with experimental literature data, except for mixtures with relatively large CH4 concentration (e.g., 8%), for which the OCV is significantly underestimated. This is expected when thermochemical reactions, in parallel to electrochemical reactions occur, since the additional H2 produced by internal steam methane reforming is not accounted within the local mixed potential model. A fuel cell polarization model is developed based on the results from the calibration procedure, and it is used to predict the polarization behavior of an SOFC fed with a H2-H2O-CO-CO2 fuel mixture. It is found that either H2 or CO may be reduced rather than oxidized via an equivalent electrochemical water-gas-shift reaction.

Regulating the surface Au sites of Au/TiO2 catalyst for achieving co-oxidation of HCHO and CO at room temperature

While supported Au catalyst is active for formaldehyde (HCHO) or CO oxidation alone at room temperature, it has not been reported hitherto to efficiently catalyze co-oxidation of HCHO and CO. In this work, we prepared the fresh Au/TiO2 (Au/Ti-F) and the Au/Ti-O by pretreating Au/Ti-F at 300 °C under O2 atmosphere. The Au/Ti-F is only active for HCHO oxidation during HCHO and CO co-oxidation, while the Au/Ti-O shows high efficiency for both HCHO and CO oxidation, achieving 100 % conversions at room temperature. Detailed characterization and DFT calculation indicate that only the Au-Ti interface sites were exposed on Au/Ti-F because other Au sites on Au NPs were covered by Au(OH)n (0 <n < 3). Over Au/Ti-O, besides the active sites at the Au-Ti interface, the metallic Au sites away from the interface were formed during the high-temperature treatment. HCHO oxidation occurs at the Au-Ti interface sites while CO oxidation occurs at the metallic top Au sites; hence high activity for HCHO and CO co-oxidation was achieved over Au/Ti-O. The obtained insight into the dual Au sites will benefit future design of supported Au catalysts.

Intermediate temperature exposure regenerates performance and active site dispersion in sintered Pd–CeO2 catalysts

Methods that can impede or reverse the effects of high-temperature sintering and deactivation are necessary to improve the utilization of catalytic materials. This work demonstrates that exposure to intermediate temperatures (800 °C) can effectively regenerate catalytic performance in sintered Pd-CeO2 catalysts, through the disintegration and partial redispersion of agglomerated Pd, which increases the abundance of sites that interface strongly with the CeO2 support. Pretreating the catalyst at 800 °C before sintering forms highly dispersed Pd species that slow the sintering of small, polycrystalline CeO2 domains. This preserves the density of Ce3+ sites that trap mobile Pd, which improves the efficacy of regeneration. The regenerated catalyst is effective in the co-oxidation of CO and C3H8, two species which are known to competitively adsorb. The simplicity of this treatment makes it attractive for on-stream regeneration of catalyst performance.

Closed-Form Formulation of the Thermodynamically Consistent Electrochemical Model Considering Electrochemical Co-Oxidation of CO and H2 for Simulating Solid Oxide Fuel Cells

Achieving efficient solid oxide fuel cell operation and simultaneous prevention of degradation effects calls for the development of precise on-line monitoring and control tools based on predictive, computationally fast models. The originality of the proposed modelling approach originates from the hypothesis that the innovative derivation procedure enables the development of a thermodynamically consistent multi-species electrochemical model that considers the electrochemical co-oxidation of carbon monoxide and hydrogen in a closed-form. The latter is achieved by coupling the equations for anodic reaction rates with the equation for anodic potential. Furthermore, the newly derived model is capable of accommodating the diffusive transport of gaseous species through the gas diffusion layer, yielding a computationally efficient quasi-one-dimensional model. This resolves a persistent knowledge gap, as the proposed modelling approach enables the modelling of multi-species fuels in a closed form, resulting in very high computational efficiency, and thus enable the model’s real-time capability. Multiple validation steps against polarisation curves with different fuel mixtures confirm the capability of the newly developed model to replicate experimental data. Furthermore, the presented results confirm the capability of the model to accurately simulate outside the calibrated variation space under different operating conditions and reformate mixtures. These functionalities position the proposed model as a beyond state-of-the-art tool for model supported development and control applications.

Modeling of two-step CO oxidation light-off on Pt/γ-Al2O3 in the presence of C3H6 and NOx

Abatement of pollutants in automotive exhaust converters can be negatively affected by interactions of reaction intermediates with the catalyst surface. This paper focuses on a specific inhibition of CO oxidation on Pt/γ-Al2O3 catalyst by C3H6 oxidation intermediates, leading to a so-called two-step CO light-off behavior. Two reaction pathways contributing to a temporary decrease of CO conversion during a temperature-programmed co-oxidation of CO and C3H6 are identified: (i) accumulation of C3H6 oxidation intermediates on the catalyst surface that block active sites and progressively inhibit both CO oxidation and total C3H6 oxidation, and (ii) CO by-product formation during propylene oxidation. The latter pathway seems to play a minor role while the inhibition effect dominates. The phenomenon is most pronounced at lower (nearly stoichiometric) oxygen concentrations and in the presence of NO. This indicates that the initial formation of C3H6 oxidation intermediates is less sensitive to O2 and NO concentrations, while the total oxidation steps depend more strongly on the actual exhaust gas composition. A novel global reaction kinetic model is proposed that captures all the experimentally observed phenomena including transient accumulation of the reaction intermediates on the catalyst surface and the resulting impact on CO and C3H6 conversion. The developed model covers a wide range of operating conditions, from three-way catalyst (stoichiometric mixture) to diesel oxidation catalyst (lean mixture).

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.

Analysis of light-off during oxidation of reactant mixtures on Pt/Al2O3 using micro-kinetic models

An elementary step mechanism, which accounts for adsorption, desorption and surface reaction steps is proposed for co-oxidation of CO, propylene, hydrogen and ethane on Pt/Al2O3. Parameter estimation is used to determine the kinetic parameters for these steps, which depend on platinum loading and dispersion. A micro-kinetic model is proposed for water-gas shift reaction that captures the experimental trend observed during co-oxidation of CO/H2. This mechanism is used to predict hysteresis features during oxidation and co-oxidation reactions. The oxidation of all the individual reactants exhibits regular hysteresis and the model predicts the experimentally observed inverse hysteresis during co-oxidation of CO and C3H6. The inverse hysteresis is caused by surface intermediates formed during C3H6 oxidation. These surface intermediates block the active sites, decreasing the light-off activity of CO during ramp down. The developed micro-kinetic model can also explain steam reforming of propylene and ethane on Pt/Al2O3. Finally, we compare the predicted hysteresis during CO oxidation by global and micro-kinetic models. The predicted ignition temperature by both global and micro-kinetic models is almost same. However, the predicted extinction temperature by the global and micro-kinetic models is rather different. This behavior is due to different rate determining step during ramp down period.

Electrochemical Oxidation of Syngas on Nickel and Ceria Anodes

Fuel flexibility of solid oxide fuel cells enables the use of low cost and practical fuels like syngas. Understanding of the oxidation kinetics with syngas is essential for proper selection of anode material and its design optimization. Using nickel and ceria pattern anodes, we study the electrochemical oxidation of syngas in both dry and wet environments. In dry environment, the polarization resistance of CO oxidation drops drastically with the addition of small amounts of hydrogen to CO gas stream. In wet environment (4 % moisture), the polarization resistance of CO is only slightly higher than syngas and hydrogen. Observation in the first case is related to the hydrogen preferential oxidation whereas latter is a combined effect of water gas shift reaction and preferential oxidation of hydrogen. Kinetic modeling is also carried out to understand hydrogen and CO co-oxidation. Simulation suggests that CO, besides hydrogen, may also electrochemically oxidize depending upon its concentration in the syngas. At higher concentration, CO electrochemical oxidation may be non-negligible especially in case of ceria anodes.

Promoting effect of TiO2 on the catalytic performance of Pt-Au/TiO2(x)-CeO2 for the co-oxidation of CO and H2 at room temperature

In the present study the promoting effect of TiO2 on the catalytic performance of Pt-Au/TiO2(x)-CeO2 catalysts for the co-oxidation of CO and H2 at room temperature has been investigated. The results showed that the addition of TiO2 to Pt-Au/CeO2 catalyst enhances the activity for the co-oxidation of CO and H2, and Pt-Au/TiO2 (10%)-CeO2 catalyst was the most active catalyst. A series of characterization methods were used to elucidate the promoting effect of TiO2. It was found that the introduction of TiO2 to Pt-Au/CeO2 led to the increase of Ce3+ and Au+ species, both of which played important roles in the co-oxidation of CO and H2. Moreover, a possible mechanism of the simultaneous removal of CO and H2 has been proposed.

Bifurcation features of mixtures containing CO and hydrocarbons in diesel oxidation catalyst

A model of a monolith reactor with radial gradients in the washcoat is used to study the ignition-extinction of pure CO, C2H6 and C3H6 and of their mixtures on a Pt/Al2O3 diesel oxidation catalyst. The ignition temperature of the individual species oxidation increases upon an increase in feed concentration of either CO or C3H6 and decreases upon an increase in feed concentration of C2H6. This is because CO, C3H6 have negative order kinetics and C2H6 has positive order kinetics. We determine the steady-state hysteresis behavior of conversion versus feed temperature during co-oxidation of CO and hydrocarbons (C3H6, C2H6). A single S-shaped bifurcation diagram of conversion versus inlet temperature is obtained during the co-oxidation of all feed mixtures of CO and C3H6 because the activation energies of CO and C3H6 are close to each other. The presence of CO in the feed inhibits the oxidation of propylene during the co-oxidation of CO and C3H6. In contrast, the ignition temperatures of pure CO and pure C2H6 are much different and CO ignites at a much lower temperature than C2H6. A double-S shaped bifurcation diagram is obtained when hysteresis exists for both CO and C2H6 during the co-oxidation. The oxidation of CO present in the feed increases the catalyst temperature and this reduces the C2H6 ignition temperature. The separated ignition is transformed to a simultaneous ignition when the CO concentration in the feed is sufficiently high. We determine the boundary in the parameter space of feed compositions and other operating conditions that separate the simultaneous and separate ignition. The application of this result for the design of DOCs for treating exhaust gases at lower inlet temperatures is discussed.

Dynamic hysteresis in monolith reactors and hysteresis effects during co-oxidation of CO and [formula omitted

In many laboratory catalytic reactors light-off curves describing the exit conversion versus inlet gas temperature do not follow the same path during ramp up and ramp down and this leads to dynamic hysteresis. This dynamic hysteresis, which may occur even under ideal conditions (e.g. plug flow and negligible heat effects) is fundamentally different from the steady-state hysteresis which is due to heat effects and thermal feedback. We present expressions for the width of the dynamic hysteresis loop as a function of the ramp rate, solid to gas heat capacity ratio, space time and the heat Peclet number, for the limiting cases of pseudo-homogeneous and two phase plug flow models with negligible heat effects (ΔTad=0). We review the conditions under which steady-state hysteresis can exist for a finite ΔTad and demonstrate the combined impact of steady-state and dynamic hysteresis effects. We study the light-off behavior and dynamic hysteresis during the co-oxidation of CO+C2H6 mixture on Pt/Al2O3. It is found that 1% of CO alone ignites at 473K whereas 500ppm of C2H6 alone ignites at 723K. The exotherm generated by the CO oxidation can reduce the ignition temperature of C2H6 during the co-oxidation of CO and C2H6. Bifurcation diagrams of solid exit temperature versus fluid inlet temperature for different inlet compositions illustrate the simultaneous as well as separate ignitions for CO and C2H6. Each 1% increase in the CO feed mole percentage decreases C2H6 ignition temperature by about 80°C. A feed of 3% CO is required for simultaneous ignition of a mixture of CO and 500ppm ofC2H6.

Steady-state and dynamic hysteresis effects during lean co-oxidation of CO and C3H6 over Pt/Al2O3 monolithic catalyst

A global kinetic model is developed for the co-oxidation of CO and C3H6 over a Pt/Al2O3 diesel oxidation monolithic catalyst based on a recent bench flow reactor study (Abedi et al., 2012). A monolith reactor model containing these kinetics is used to elucidate the dynamic and steady state hysteresis behavior observed during oxidation of a CO+C3H6 mixture on Pt/Al2O3. Dynamic hysteresis is observed during temperature-programmed oxidation which involves ramping the feed gas temperature up and down at a constant ramp rate. The predicted solid temperature spatial profile during ramp-up and ramp-down shows how the catalyst temperature lags behind the inlet temperature change, especially during the ramp-down. The dynamic hysteresis loop, whose width increases with the ramp rate and effective heat Peclet number (Peh,eff), is mainly a result of disparate time scales of thermal front propagation and temperature ramp and will be present even for reactions with negligible heat release. Steady state hysteresis can also be explained based on adiabatic temperature rise (ΔTad) and Peh,eff, i.e. thermokinetic multiplicity. The region of steady state hysteresis loop expands with an increasing ΔTad which is proportional to the feed concentrations of CO and C3H6. For fixed ΔTad, the steady state multiplicity disappears as Peh,eff increases or when there is negligible thermal feedback due to intra or interphase heat transfer. Finally, the model is used to construct a map that may be used to assess the light-off features of multiple reaction systems in monolith reactors.

Complete methanol oxidation in carbon monoxide streams over Pd/CeO2 catalysts: Correlation between activity and properties

Three different techniques, a hydrothermal process with impregnation, solution-combustion and common precipitation with impregnation, were employed for the synthesis of 1.5wt% Pd/CeO2, and the resulting catalysts were denoted as Pd/HY, Pd/COM and Pd/PI, respectively. These catalysts have been investigated in terms of catalytic co-oxidation of CO and methanol and characterized by means of XRD, BET, SEM, STEM-EDS, CO chemisorption, XPS, Raman, O2-TPD, H2-TPR and CO-TPD measurements. Activity results show that a rapid catalytic oxidation of CO following the Mars-van Krevelen-type mechanism favors the complete methanol oxidation at low temperature. Consequently, the Pd/HY catalyst with the highest degree of defect sites connected with the mobility of activated oxygen shows the best catalytic performance. And the generation of more defect sites demonstrated by Raman is assumed for the low crystallinity and small crystallite size of CeO2 synthesized through a hydrothermal process while the high mobility of activated oxygen is evidenced by O2-TPD and H2-TPR. Additionally, the outstanding catalytic performance of Pd/HY is also associated with its high Pd dispersion and stable PdOx species as well as good redox behavior.

Catalytic co-oxidation of CO and H2 over FeOx-supported Pd catalyst at low temperatures

Co-oxidation of CO and H2 or oxidation of H2 in the presence of CO was first achieved at low temperatures (0–20°C) over single-component catalysts (Pd/FeOx) at a space velocity of 15,000mlgcat-1h-1. A systematical study of FeOx- and Al2O3-supported Pd catalysts is carried out by means of STEM, TEM, XRD, XPS, Mössbauer spectra, and time-resolved CO titration. Highly dispersed Pd nanoparticles with size 1–1.8nm over 5.4wt% Pd/FeOx are confirmed by HAADF-STEM and HRTEM. Time-resolved CO titration, Mössbauer spectra, and XRD and XPS results suggest FeOx support supplying active oxygen is involved in the oxidations. Particle size effect and oxygen supply from FeOx support are speculated to be the main reasons for the excellent performance of Pd/FeOx in co-oxidation of CO and H2 or oxidation of H2 in the presence of CO.

Kinetic and bifurcation analysis of the cooxidation of CO and [formula omitted] in catalytic monolith reactors

Existing micro and global reaction kinetic models are not able to capture some of the key experimentally observed light-off trends of the CO – H 2 – O 2 system in a monolithic reactor. We modify the literature microkinetic mechanisms to correctly predict the H 2 light-off at room temperature in the absence of CO, selectivity towards CO oxidation before the simultaneous light-off of CO and H 2 , the enhancement effect of H 2 , and the inhibiting effect of CO on light-off in a CO – H 2 – O 2 system. To explain the effect of H 2 on light-off behavior of CO, we have proposed the formation and decomposition of a H–CO complex on Pt. This results in a decrease of CO desorption activation energy with an increase in the coverage of H species on Pt, thus assisting in the creation of more sites for oxygen adsorption. Based on the microkinetic studies and experimental results available in the literature, modifications have also been made to the global reaction kinetics, which currently do not distinguish between CO and H 2 oxidation kinetics, and also do not contain the effect of H 2 on CO oxidation kinetics. We propose global kinetics with composition dependent activation energies to explain the experimentally observed light-off behavior. A bifurcation analysis of a short monolith reactor model has been conducted to study the effect of inlet temperature, inlet concentrations and the exposed Pt surface area on the light-off and multiplicity behavior. A comparison of the light-off curves generated using the global model and microkinetic model is done and the effect of rate parameters on the region of multiplicity is investigated.

Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO).

Production and consumption processes in soils contribute to the global cycles of many trace gases (CH4, CO, OCS, H2, N2O, and NO) that are relevant for atmospheric chemistry and climate. Soil microbial processes contribute substantially to the budgets of atmospheric trace gases. The flux of trace gases between soil and atmosphere is usually the result of simultaneously operating production and consumption processes in soil: The relevant processes are not yet proven with absolute certainty, but the following are likely for trace gas consumption: H2 oxidation by abiontic soil enzymes; CO cooxidation by the ammonium monooxygenase of nitrifying bacteria; CH4 oxidation by unknown methanotrophic bacteria that utilize CH4 for growth; OCS hydrolysis by bacteria containing carbonic anhydrase; N2O reduction to N2 by denitrifying bacteria; NO consumption by either reduction to N2O in denitrifiers or oxidation to nitrate in heterotrophic bacteria. Wetland soils, in contrast to upland soils are generally anoxic and thus support the production of trace gases (H2, CO, CH4, N2O, and NO) by anaerobic bacteria such as fermenters, methanogens, acetogens, sulfate reducers, and denitrifiers. Methane is the dominant gaseous product of anaerobic degradation of organic matter and is released into the atmosphere, whereas the other trace gases are only intermediates, which are mostly cycled within the anoxic habitat. A significant percentage of the produced methane is oxidized by methanotrophic bacteria at anoxic-oxic interfaces such as the soil surface and the root surface of aquatic plants that serve as conduits for O2 transport into and CH4 transport out of the wetland soils. The dominant production processes in upland soils are different from those in wetland soils and include H2 production by biological N2 fixation, CO production by chemical decomposition of soil organic matter, and NO and N2O production by nitrification and denitrification. The processes responsible for CH4 production in upland soils are completely unclear, as are the OCS production processes in general. A problem for future research is the attribution of trace gas metabolic processes not only to functional groups of microorganisms but also to particular taxa. Thus, it is completely unclear how important microbial diversity is for the control of trace gas flux at the ecosystem level. However, different microbial communities may be part of the reason for differences in trace gas metabolism, e.g., effects of nitrogen fertilizers on CH4 uptake by soil; decrease of CH4 production with decreasing temperature; or different rates and modes of NO and N2O production in different soils and under different conditions.

Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO).

Production and consumption processes in soils contribute to the global cycles of many trace gases (CH4, CO, OCS, H2, N2O, and NO) that are relevant for atmospheric chemistry and climate. Soil microbial processes contribute substantially to the budgets of atmospheric trace gases. The flux of trace gases between soil and atmosphere is usually the result of simultaneously operating production and consumption processes in soil: The relevant processes are not yet proven with absolute certainty, but the following are likely for trace gas consumption: H2 oxidation by abiontic soil enzymes; CO cooxidation by the ammonium monooxygenase of nitrifying bacteria; CH4 oxidation by unknown methanotrophic bacteria that utilize CH4 for growth; OCS hydrolysis by bacteria containing carbonic anhydrase; N2O reduction to N2 by denitrifying bacteria; NO consumption by either reduction to N2O in denitrifiers or oxidation to nitrate in heterotrophic bacteria. Wetland soils, in contrast to upland soils are generally anoxic and thus support the production of trace gases (H2, CO, CH4, N2O, and NO) by anaerobic bacteria such as fermenters, methanogens, acetogens, sulfate reducers, and denitrifiers. Methane is the dominant gaseous product of anaerobic degradation of organic matter and is released into the atmosphere, whereas the other trace gases are only intermediates, which are mostly cycled within the anoxic habitat. A significant percentage of the produced methane is oxidized by methanotrophic bacteria at anoxic-oxic interfaces such as the soil surface and the root surface of aquatic plants that serve as conduits for O2 transport into and CH4 transport out of the wetland soils. The dominant production processes in upland soils are different from those in wetland soils and include H2 production by biological N2 fixation, CO production by chemical decomposition of soil organic matter, and NO and N2O production by nitrification and denitrification. The processes responsible for CH4 production in upland soils are completely unclear, as are the OCS production processes in general. A problem for future research is the attribution of trace gas metabolic processes not only to functional groups of microorganisms but also to particular taxa. Thus, it is completely unclear how important microbial diversity is for the control of trace gas flux at the ecosystem level. However, different microbial communities may be part of the reason for differences in trace gas metabolism, e.g., effects of nitrogen fertilizers on CH4 uptake by soil; decrease of CH4 production with decreasing temperature; or different rates and modes of NO and N2O production in different soils and under different conditions.