Activation Energy For CO Oxidation Research Articles

Preferential oxidation of CO in H 2-rich feeds over mesoporous copper manganese oxides synthesized by a redox method

Mesoporous copper manganese oxides with high surface areas (>268 m 2/g) were prepared using the redox method and tested in the preferential oxidation of CO. These materials were highly active and selective under typical operating conditions of a proton-exchange membrane fuel cell. The synthesized catalysts preferentially oxidized CO with a stoichiometric amount of oxygen in the feed gas. The presence of CO 2 and H 2O in the feed gas retarded catalytic activity significantly at low (<90 °C) temperatures. The catalysts showed stable activity in long-term (12 h) experiments with realistic feeds. The high catalytic activity was attributed to a combination of factors, including high surface area, low crystallinity, low activation energy for CO oxidation, compositional homogeneity of the copper manganese oxides, and the presence of readily available lattice oxygen for CO oxidation. The high selectivity (100% with stoichiometric reactants) was ascribed to the lower activation energy for CO oxidation compared to the activation energy for H 2 oxidation.

Carbon Monoxide Oxidation over Au/Pd(100) Model Alloy Catalysts

The oxidation of carbon monoxide is studied on a series of Au/Pd(100) alloys with various compositions that are formed by adsorbing four monolayers of gold onto a Pd(100) surface and by heating to form the alloy. The composition of the alloy surface is measured using Auger and low-energy ion scattering spectroscopies and the surface chemistry followed by temperature-programmed desorption and reflection absorption infrared spectroscopy. Carbon monoxide oxidized rapidly on Au/Pd(100) model alloy surfaces. Oxygen is found to dissociatively adsorb on alloy surfaces for gold coverages below ∼0.6 ML and relatively stable molecular oxygen species were found on the surface at gold coverages between ∼0.2 and 0.6 ML, but did not contribute to CO oxidation on these Au/Pd(100) alloys. Carbon monoxide oxidation and desorption were found to occur at almost identical temperatures, implying that the activation energies for these processes are almost identical. This suggests that the activation energy for CO oxidation is ...

Sonochemical synthesis of Ce1−xFexO2−δ (0 ≤x≤ 0.45) and Ce0.65Fe0.33Pd0.02O2−δ nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst

Nanocrystalline Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) of ~4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe(3+) ion in CeO(2), lattice oxygen is activated and 33% Fe substituted CeO(2)i.e. Ce(0.67)Fe(0.33)O(1.835) reversibly releases 0.31[O] up to 600 °C which is higher or comparable to the oxygen storage capacity of CeO(2)-ZrO(2) based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd(2+/0)(0.89 V) and Fe(3+/2+) (0.77 V) with Ce(4+/3+) (1.61 V), Pd ion accelerates the electron transfer from Fe(2+) to Ce(4+) in Ce(0.65)Fe(0.33)Pd(0.02)O(1.815), making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) is found to be as low as 38 kJ mol(-1). Ce(0.67)Fe(0.33)O(1.835) and Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) have also shown high activity for the water gas shift reaction. CO conversion to CO(2) is 100% H(2) specific with these catalysts and conversion rate was found to be as high 27.2 μmoles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce(0.65)Fe(0.33)Pd(0.02)O(1.815).

Activity, stability, and deactivation behavior of supported Au/TiO 2 catalysts in the CO oxidation and preferential CO oxidation reaction at elevated temperatures

The influence of elevated reaction temperatures on the activity, long-time stability, and deactivation behavior of Au/TiO 2 catalysts for CO oxidation was studied by kinetic and in situ IR measurements in the temperature range 80–180 °C, both in an idealized, H 2-free reaction atmosphere and in a H 2-rich gas mixture. The results are discussed in a molecular picture. Most important for practical applications (PROX reaction), the selectivity for CO oxidation in H 2-rich gas mixtures decreases sharply with increasing temperature, from ∼60% at 80 °C to ∼15% at 180 °C. The increasing tendency for H 2 oxidation is reflected also by a distinctly lower apparent activation energy for CO oxidation in the presence of H 2 (18 ± 3 kJ mol −1) compared to reaction in a H 2-free reaction mixture (30 ± 3 kJ mol −1). In all cases, carbonate formation is observed and proposed to contribute to the deactivation, but with different extents depending on the reaction conditions.

Activity Enhancement of WO3-Promoted Ir/SiO2 Catalysts by High-Temperature Calcination for the Selective Reduction of NO with CO

Abstract The catalytic activity of WO3-promoted Ir/SiO2 for the selective reduction of NO with CO in the presence of excess O2 is strongly dependent on the catalyst calcination conditions in the preparation process. WO3/Ir/SiO2 catalyst prepared by air calcination showed higher activity than that prepared by H2 reduction. The activity of WO3/Ir/SiO2 catalyst prepared by air calcination was also found to increase with increasing calcination temperature up to 1073 K; however, a further increase in the temperature to 1173 K caused a decrease in the NO reduction activity, although very high selectivity was observed for this catalyst. The lack of a major difference in the apparent activation energy for NO reduction suggests that NO reduction occurs at the same reaction sites, irrespective of calcination temperature. On the other hand, the apparent activation energy for CO oxidation increased with increased calcination temperature, suggesting that the reaction sites for CO oxidation change according to calcination temperature. Catalyst characterizations revealed that iridium species interacting strongly with WO3 are preferentially created by high-temperature air calcination. A combination of partially reduced iridium species (IrO2−x) and WO3 (IrO2−x–WO3) was concluded to act as catalytically active sites of WO3/Ir/SiO2 for NO reduction.

Investigation of CO Oxidation Transient Kinetics on an Oxygen Pre-covered Au(211) Stepped Surface

The desire to explain the origin(s) of the unexpected catalytic activity of oxide-supported Au nanoparticles for CO oxidation discovered by Haruta and coworkers has stimulated numerous experimental and theoretical studies of Au nanoclusters in the gas phase and on metal oxide supports, and on Au single-crystal surfaces. In order to explore further the reactivity of low-coordination Au step sites, we have performed transient kinetics studies of CO oxidation on an O-precovered, stepped Au(211) single crystal surface. We found behavior similar to that observed previously on flat Au(111) and (110) surfaces; i.e., there is no evidence in these transient kinetics for any special reactivity associated with this stepped Au surface. The CO oxidation reaction rate was highly dependent on the initial oxygen coverage, and we determined an apparent activation energy for CO oxidation of −7.0 kJ mol−1 for θ O init = 0.9 ML. Within the Langmuir-Hinschelwood (LH) reaction scheme, we estimate an activation energy of E LH = 20–43 kJ mol−1 on this surface for CO oxidation via this pathway. This is somewhat below the value of 67 kJ mol−1 predicted by recent theoretical calculations.

Size and pressure independent kinetics of CO oxidation on alumina‐supported iridium nanoparticles

AbstractThe gas‐phase oxidation of CO catalyzed by iridium was studied under CO‐rich reactant conditions over the pressure range 10−4–10−2 Pa to investigate the controversial size and pressure dependencies. The reactivity of iridium clusters with an average size of 3 nm was compared to macroscopic metal particles with extended single crystal facets over a range of temperatures (433–573 K), and the apparent activation energies for CO oxidation were consistent with the known activation energies for CO desorption from iridium surfaces. No size‐specific dependence of the reaction kinetics was measured, and no indication of the previously reported “pressure gap” was observed. Unlike many other transition metal catalysts, iridium may be used as a model catalyst for CO oxidation in different morphological forms over a range of pressures without invoking a change in the kinetic model. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 826–830, 2008

Partial pressure dependent in situ spectroscopic study on the preferential CO oxidation in hydrogen (PROX) over Pt/ceria catalysts

Platinum supported on ceria can oxidize CO in excess hydrogen selectively (PROX process). In situ DRIFTS and high-pressure (∼1 mbar) XPS experiments were performed to study the mechanism of the PROX reaction on Pt/ceria catalysts. The partial pressure of O2 and/or CO was varied and correlated with induced changes in activity and selectivity as well as with the surface state and species under reaction conditions. Pt-carbonyl species changed rather insignificantly, especially relative to the wide variations of the product pattern with changing feed composition. Furthermore, the interconversion of formate and carbonate species was observed. Therefore, the changes in the evolution of surface species detected by in situ DRIFTS cannot explain the variation observed in CO oxidation activity. On the other hand, high-pressure XPS showed significant modification of the surface state with changing feed composition. Most significantly, oxygen vacancy formation seemed to correlate with enhanced CO oxidation activity. At higher vacancy density, water desorption was hindered. Highly hydrated ceria with significant vacancy density was found to be beneficial for the PROX process; here surface water blocked Hads oxidation sites. Moreover, lower apparent activation energy of CO oxidation was measured in the PROX reaction on catalysts with more vacancies. The results given here reinforce the view of catalysts being adaptive to a certain reaction rather than having active sites as prepared. Whereas IR-detectable surface species may only be indicators and/or consequences of this surface change, formation of the beneficial surface/near-surface state may be the rate-limiting factor in several catalytic processes.

Kinetics of catalytic oxidation of CO over copper-manganese oxide catalyst

The hopcalite-like catalyst was prepared by co-precipitation, which showed excellent activity and stability for CO oxidation at room-temperature. The activity of carbon monoxide oxidation was investigated at different temperature. The kinetics of carbon monoxide oxidation over copper-manganese oxides catalysts was obtained by means of recycling batch reactor. As a result, CO oxidation followed different reaction mechanism at a broad range of the experimental temperature. The reaction activation energy of CO oxidation was found to be 64.08 kJ mol −1 for 273–313 K and 12.15 kJ mol −1 for 333–373 K, respectively.

The effect of fractal dimension of porous catalysts on the activation energy of CO oxidation

An approach based on the interrelationship between the activation energy of the rate-limiting step of a heterogeneous catalytic reaction and the structure of the catalyst surface expressed in terms of fractal geometry is presented for the example of the oxidation of carbon monoxide. The activation energy of this reaction was shown to increase with increasing nonuniformity of the catalyst surface given by the fractal dimension DF.

Single step direct coating of 3-way catalysts on cordierite monolith by solution combustion method: High catalytic activity of Ce 0.98Pd0.02O2-δ

A new process of coating of active exhaust catalyst over $\gamma-Al_2O_3$ coated cordierite honeycomb is reported here. The process consists of (a) growing $\gamma-Al_2O_3$ on cordierite by solution combustion of $Al(NO_3)_3$ and oxylyldihydrazide (ODH) at $600^oC$ and active catalyst phase $Ce_{0.98}Pd_{0.02}O_2-\delta$ on $\gamma-Al_2O_3$ coated cordierite again by combustion of ceric ammonium nitrate and ODH with $1.2\times 10^{-3} M PdCl_2$ solution at $500^oC$. Weight of active catalyst can be varied from 0.02 to 2 wt% which is sufficient but can be loaded even up to 12 wt% by repeating dip dry combustion. Adhesion of catalyst to cordierite surface is via oxide growth which is very strong. About 100% conversion of CO is achieved below $80^oC$ at a space velocity of 880$h^-1$. At much higher space velocity of 21,000 $h^-1$, 100% conversion is obtained below $245^oC$. Activation energy for CO oxidation is 8.4 kcal/mol. At a space velocity of 880 $h^-1$ 100% NO conversion is attained below $185^oC$ and 100% conversion of 'HC' $(C_2H_2)$ below $220^oC$. At same space velocity 3-way catalytic performance over $Ce_{0.98}Pd_{0.02}O_{2-\delta}$ coated monolith shows 100% conversion of all the pollutants below $220^oC$ with 15% of excess oxygen. In this method, handling of nano material – powder is avoided.

In-situ soft X-ray studies of CO oxidation on the Pt(111) surface

Temperature-programmed and isothermal carbon–K-edge fluorescence yield measurements have been used to characterize carbon monoxide oxidation on the Pt(1 1 1) surface in pressures of flowing oxygen ranging from 0.00002 to 0.02 Torr over the 200–600 K temperature range. Temperature programmed-fluorescence yield near edge spectroscopy (TP-FYNES) of the oxidation of pre-adsorbed CO coverages in O2 pressures up to 0.02 Torr has been used to directly characterize surface reaction rates and to focus on the effects of temperature and oxygen flux during oxidation. For a saturated CO coverage, oxidation does not begin until significant CO desorption from the surface occurs. This finding is supported by the observation that the onset temperature for oxidation remains at 305 K for the CO saturated surface even for O2 pressures up to 0.02 Torr. For large CO coverages below full saturation, the onset temperature for oxidation decreases with increasing oxygen pressure, indicating oxygen adsorption is not completely inhibited. After oxidation begins for large CO coverages, oxidation rapidly decreases the CO surface concentration, as the activation energy for CO oxidation is smaller than the activation energy for CO desorption. Isothermal oxidation experiments clearly show that the oxidation activation energies observed depend on both the CO coverage and the O2 pressure. Oxidation activation energies are larger for small CO coverages, with a clear increase below 70% of a saturated CO coverage. For instance, at an oxygen pressure of 0.002 Torr, the activation energy for oxidation increases from 11.9 to 16.8 kcal/mol as the CO coverage decreases below 70% of a saturated coverage. In addition, oxidation activation energies decrease with increasing oxygen pressure. For example, as the oxygen pressure is increased from 0.002 to 0.02 Torr, the activation energy for oxidation of CO coverages below 70% of a saturated coverage is decreased from 16.8 to 12.3 kcal/mol. For the fully saturated CO coverage, the inhibition of oxygen adsorption completely suppresses oxidation below the CO desorption temperature. For large CO coverages below saturation where oxidation proceeds at a measurable rate, the order in oxygen is approximately zero (0.08), suggesting that oxidation is generally not limited by oxygen adsorption. Taken together, these findings clearly confirm that CO inhibition of O2 adsorption is crucial for the saturated coverage of CO. For CO coverages below saturation, interactions within the adsorbed layer significantly alter both the activation energy and pre-factor for CO oxidation on the low-index Pt(1 1 1) surface.

Structure sensitivity in the CO oxidation on rhodium: Effect of adsorbate coverages on oxidation kinetics on Rh(100) and Rh(111)

Temperature-programmed reaction spectroscopy has been used to study the surface reaction between CO and O-atoms on Rh(100) and Rh(111) at a range of different adsorbate coverages. Comparison of the reaction on both surfaces in the low coverage regime, where the kinetics can be described by a straightforward Langmuir–Hinshelwood mechanism reveals that the CO oxidation is structure sensitive, with the rate constant being an order of magnitude higher on the Rh(100) than on the Rh(111) surface. As a consequence, the selectivity of the CO+O reaction to CO2 is about 100% on Rh(100), whereas on Rh(111) the oxidation reaction competes with CO desorption. At low CO coverage, CO oxidation is an elementary step on Rh(100) for a broad range of oxygen coverages. We report kinetic parameters Ea=103±5 kJ/mol and ν=1012.7±0.7 for θO=θCO→0 on Rh(100). The activation energy for CO oxidation on Rh(100) decreases continuously with increasing O-coverage. At low coverage (θO&amp;lt;0.25 ML) we attribute this to destabilization of CO, leading to an increase in the CO2 formation rate. At higher coverage (θO&amp;gt;0.25 ML) O-atoms become destabilized as well, as lateral interactions between O-atoms come into play at these coverages. The interactions result in a greatly enhanced rate of reaction at higher coverages.

Kinetics of low temperature CO oxidation over Pt/SnO2 catalyst

In a CO−O2 stoichiometric mixture, the kinetic parameters, reaction order, rate constant and activation energy of CO oxidation over a Pt/SnO2 catalyst have been measured using a fixed bed flow reactor near 0°C. The results show that it is a first-order reaction. The activation energy of CO oxidation over Pt/SnO2 prepared with SnO2 calcined at 300°C was approximately 21 kJ/mol. The activation energy of CO oxidation over Pt/SnO2 changed slowly with SnO2 calcination temperature above 400°C, and reached approximately 45 kJ/mol.

Low-Temperature Oxidation of CO over Gold Supported on TiO 2, α-Fe 2O 3, and Co 3O 4

Gold can be highly dispersed on a variety of metal oxides by coprecipitation and deposition-precipitation followed by calcination in air. The small gold particles are hemispherical in shape and stabilized by epitaxial contact, dislocations, or contact with an amorphous oxide layer. Such supported gold differs in catalytic nature from unsupported gold particles and exhibits high catalytic activities for low-temperature oxidation of CO. Especially, gold supported on TiO 2, α-Fe 2O 3, Co 3O 4, NiO, Be(OH) 2, and Mg(OH) 2 is very active even at temperatures below 0°C. Among the gold catalysts supported on TiO 2, α-Fe 2O 3, and Co 3O 4 the turnover frequencies for CO oxidation per surface gold atom are almost independent of the kind of support oxides used and increase sharply with a decrease in diameter of gold particles below 4 nm. Small gold particles not only provide the sites for the reversible adsorption of CO but also appreciably increase the amount of oxygen adsorbed on the support oxides. In the temperature range −10 to 65°C, the activation energies for CO oxidation were 8.2 kcal/mol (Au/TiO 2), 8.4 kcal/mol (Au/α-Fe 2O 3), and 3.9 kcal/mol (Au/Co 3O 4). The rate of CO oxidation is zero order with respect to CO for the three catalysts, and 0.2-0.3 for Au/TiO 2 and Au/Co 3O 4 and zero order for Au/α-Fe 2O 3 with respect to O 2. By taking into consideration TPD and FT-IR data, a mechanism is proposed in which CO adsorbed on gold particles migrates toward the perimeter on support oxides and there it reacts with adsorbed oxygen to form bidentate carbonate species. The decomposition of the carbonate intermediate is considered to be rate-determining.

Characterization of supported Pd catalysts

AbstractSupported Pd catalysts are active for the oxidation of carbon monoxide at low temperature. To understand the catalytic action on a molecular basis, catalysts with different metal loadings have been investigated by activity tests with regard to CO2 formation, by CO chemisorption, N2 physisorption and XPS measurements. The rate of CO2 formation turns out to be independent of CO partial pressure and dependent on O2 partial pressure (0.3 order) in the pressure and temperature range investigated. The activation energy of CO oxidation has a value of 40 kJ mol−1 independent of metal loading. The specific surface are and the pore structure of the support is not significantly changed even by a metal loading as high as 17 weight per cent. The irreversibly adsorbed amount of CO—determined at 273 K in a vacuum microbalance—is linearly related to the CO2 formation rate. The dispersion determined by XPS measurements is proportional to that calculated from the CO chemisorption results.

Oxidation of carbon monoxide on rare earth cobaltites — Role of spin state equilibrium

The catalytic oxidation of CO on a series of LnCoO3 compounds (where Ln=La, Pr, Nd, Sm, Eu, Gd, Dy or Ho) has been studied. The Arrhenius plots show gradient changes around 180–200 °C which is the temperature range wherein ordering of low spin and high spin states starts. The activation energy for CO oxidation in the low temperature region varies linearly with oxygen deficiency. IR-Spectra of the adsorbed species in both the high and low temperature regions show the presence of carbonate species.

A study of the influence of thermal treatment of the catalyst on the catalytic oxidation of CO on Co 3O 4

Various Co 3O 4 catalyst samples were obtained by thermal decomposition of basic cobalt carbonate in air in the temperature range 250 – 900 °C. The reaction between CO and oxygen on the surface of these catalysts was carried out at various temperatures between 80 and 150 °C. The catalytic activity of cobaltic oxide samples was found to decrease monotonically with increase in their preparation temperature from 250 to 500 °C. The solid retained its catalytic activity on thermal treatment at 700 °C but underwent a sudden decrease in activity when it was calcined in air at 900 °C. These results are attributed to the evolution of the specific surface area of Co 3O 4 catalysts with thermal treatment and the partial reduction of Co 3O 4 to CoO at 900 °C. The apparent activation energy of CO oxidation in the temperature range 80–125 °C in contact with Co 3O 4 prepared at 250, 300 a nd 400 °C was about 2 kcal mol -1. For Co 3O 4 prepared at 500 °C (Co 3O 4(500 °C)) it attained 6.5 kcal mol -1 at 95–150 °C. The mechanism of catalytic oxidation of CO on cobaltic oxide catalysts prepared at 250, 300 and 400 °C was different from the dominant mechanism on Co 3O 4(500 °C).