The effects of oscillatory feeding of CO and O2 on the performance of a monolithic catalytic converter of automobile exhaust gas: a modelling study

A monolithic catalytic converter of automobile exhaust gas was modelled in order to assess the effects of oscillatory feeding on the performance of the reactor with respect to CO oxidation by O₂. Simulations were performed with an oscillating feed composition of CO and O₂. The influence of frequency, amplitude, phase angle and ratio of reactants in the feed on the time average CO conversion was investigated. An improvement relative to the steady state conversion of 10% maximum is obtained at temperatures below the light-off temperature, at frequencies below 0.1 Hz and an amplitude of 15%. The reverse effect is obtained from temperatures slightly above the light-off temperature upwards. These effects are strongest when CO and O₂ oscillate in counterphase. The explanation for this effect is given in terms of strongly changing surface coverage during cycling of the feed concentrations.     

Impedance study of methanol oxidation on platinum electrodes

Methanol oxidation on Pt electrodes is studied by ac voltammetry. Data from voltammograms at frequencies from 0.5 Hz to 20 kHz are assembled into electrochemical impedance spectra and analysed using equivalent circuits. Inductive behavior and negative relaxation times are attributed to nucleation and growth behavior. The rate-determining step is proposed to be the reaction of adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites.     

Induced low temperature catalytic ignition by transient changes in the gas composition

The effect of gas composition changes on the low temperature activity for supported platinum model catalysts has been studied. By introducing well-controlled periodic O₂ pulses to a simple diluted gas mixture of CO and O₂, a substantial improvement of the low temperature oxidation activity was observed. The reason for low activity on noble metals at low temperatures is often attributed to self-poisoning by CO. The improved catalytic performance observed is proposed to origin from the transients causing a surface reactant composition that is favourable for the reaction rate, i.e. lower degree of self-poisoning. This was also confirmed by in situ Fourier transform infrared (FT-IR) spectroscopy in combination with mass spectrometry (MS) measurements, which gave evidence for the existence of a strong interplay between the gas phase concentration and the adsorbate composition for these catalysts.     

Insights into the dynamics of oxygen storage/release phenomena in model ceria–zirconia catalysts as inferred from transient studies using H2, CO and soot as reductants

Samples of ceria–zirconia pre-treated under various conditions have been used as catalysts in CO and soot oxidation under stationary and transient conditions, in the presence and in the absence of oxygen. Their behaviour has been compared with that observed under redox conditions in the presence of hydrogen (oxygen storage activity). All the catalysts are active in CO and soot oxidation. Under stationary conditions, the activity in CO oxidation depends on the amount of Ce present, with little contribution from the redox capacity of the support and is strongly influenced by surface area. When the reaction is carried out under transient conditions, especially with low-surface area samples, the performances of ceria–zirconia are higher than those of ceria, with a maximum in the middle composition range. Interestingly, a similar behaviour is observed in soot combustion, where the activity for low-surface area sample is dependent on composition. This suggests that oxygen from the support plays a key role also in the oxidation of large carbon particles under a fully oxidizing mixture.     

低温氧化型过渡金属氧化物催化剂研究进展

汽车冷启动时催化剂床层温度低,尾气中的CO和烃类不能被传统三效催化剂有效消除。目前,非贵金属类的过渡金属氧化物催化剂用于CO和烃类的低温氧化受到了广泛关注。本文综述了近几年来国内外以Cu、Co和Mn的氧化物为主要代表的过渡金属氧化物上烃类和CO氧化的研究进展,对催化剂上界面氧空位参与氧化过程的反应机理进行了总结,展望了过渡金属氧化物催化剂用于CO和烃类低温氧化的未来研究趋势。     

富氢气氛中CO优先氧化CuO-CeO_2基催化剂的研究进展

CuO-CeO₂基催化剂对富氢气氛中CO优先氧化具有优良的活性和选择性,是替代贵金属最具潜力的催化体系之一。本文系统总结了CO优先氧化用CuO-CeO₂基催化剂的研究现状,着重阐述催化剂制备工艺、成分优化、助剂改性、载体调变等对催化性能的影响规律,介绍催化活性位与CO优先氧化的催化机理,探讨催化剂稳定性与失活机制等,同时指出该催化体系目前存在的主要问题,并展望了今后一段时期可能的发展趋势。     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Research on unsupported nanoporous gold catalyst for CO oxidation

Nanoporous gold (NPG), a novel unsupported gold catalyst prepared by dealloying, exhibits exceptional catalytic activity for CO oxidation. Systematic studies were carried out on this new catalytic system, including the active sites of catalysts, the reaction kinetics, and activity dependence as functions of space velocity and temperature. Our results show strong evidence that metallic gold atoms on NPG are the intrinsic active sites at which the reaction of CO with O₂ occurs. The kinetic study found that the reaction rate of CO oxidation on unsupported NPG depends significantly on CO concentration but only slightly on O₂ concentration. We suggest that CO adsorption plays a decisive role in CO oxidation on NPG as the rate-limiting step. By completely ruling out the support influence, our findings provide considerable insight into the role of gold catalysts.     

Ruthenium as oxidation catalyst: bridging the pressure and material gaps between ideal and real systems in heterogeneous catalysis by applying DRIFT spectroscopy and the TAP reactor

Two supported Ru catalysts were prepared by the chemical vapor deposition of Ru₃(CO)₁₂ on MgO and SiO₂ (MOCVD). TEM, XRD, and static H₂ chemisorption measurements confirmed that the Ru particle size was about 2 nm on both supports. Using in situ DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy at atmospheric pressure it was found that the adsorption of CO on the reduced samples is clearly influenced by the supports whereas the adsorption of CO on the oxidized Ru catalysts is essentially independent of the support. O₂ chemisorption measurements showed that a thin RuO₂ surface layer was formed on both catalysts under oxidizing conditions at room temperature. The observed C–O stretching frequencies were found to be in good agreement with HREELS and LEED data reported for the RuO₂(1 1 0) single crystal surface. The catalytic activity was assessed under high-vacuum conditions using the TAP (temporal analysis of products) reactor by co-feeding CO and O₂. These conditions ensured that heat and mass transfer limitations were absent. Both supported Ru catalysts were found to be highly active and stable under the CO oxidation conditions even down to room temperature. The deactivation of the catalysts observed at room temperature was reversible and independent of the support. The turnover frequencies (number of CO₂ molecules per metal surface site per second) derived from steady-state measurements are in good agreement with data reported for the RuO₂(1 1 0) single crystal surface under UHV conditions. Based on the results of the DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) and the kinetic measurements supported RuO₂ is identified as the catalytically active phase. In addition, the turnover frequencies are in good agreement with data reported for Ru/SiO₂ at atmospheric pressure. Thus, both the materials and the pressure gap were bridged successfully.     

Synergism and kinetics in CO oxidation over palladium-rhodium bimetallic catalysts

The activity and kinetics of CO oxidation over alumina-supported Pd-Rh bimetallic catalysts were investigated. One bimetallic catalyst, Pd-Rh(2), was prepared by two-step impregnation and another, Pd-Rh(l), by simultaneous impregnation. Monometallic catalysts as well as a physical mixture of them were also prepared. The catalysts were characterized by selective chemisorption of both H₂ and CO, and an attempt was made to determine the surface compositions of the bimetallic catalysts. The bimetallic catalysts showed different kinetic behavior, such as higher turnover frequencies (TOFs), lower apparent activation energies and/or negative reaction orders for CO which were smaller in the absolute value, from that of the monometallic catalysts as well as a physical mixture of them. It is suggested that this Pd-Rh synergism is due to an interaction on the catalyst surface, such that adsorbed CO or oxygen on one metal migrates to the other metal site so that the reaction rate is facilitated and also that the particles of Pd and Rh are located close enough to each other for the interaction to occur. On the surface of Pd-Rh (2) most of the Pd and Rh particles existed as separate entities, while a great portion of the particles on Pd-Rh(l) exhibited the surface enrichment of Pd. This explains the higher TOF and the negative reaction orders for CO over Pd-Rh(2) which were smaller in the absolute value than those over Pd-Rh(l).     

The effects of SO2 on the oxidation of CO and propane on supported Pt and Au catalysts

The catalytic activities of Pt and Au supported on TiO₂ were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO₂ was observed only over the TiO₂-supported Au catalyst, indicating that SO₂ can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts.     

ZrO2负载铜镍双金属对CO低温催化活化研究

用两种方法合成载体ZrO2,分别负载镍单金属和铜镍双金属制备催化剂,对其做CO催化,确定载体最佳制备方法。用最佳方法制备的载体负载铜镍金属做催化剂,考察催化剂焙烧温度和负载比例不同对CO催化影响。实验结果表明,用溶胶凝胶超临界干燥法制备的ZrO2为载体负载铜镍制备的催化剂其最佳焙烧温度为250℃,Zr与Cu和Ni的最佳物质的量比是10：8：4时,获得催化剂起始催化活性温度为22℃,169℃时C0的转化率达到50％,较好地实现了ZrO2负载铜镍在较低温度下对CO的催化。     

The electro-oxidation of carbon monoxide, hydrogen/carbon monoxide and methanol in acid medium on Pt-Sn catalysts for low-temperature fuel cells: A comparative review of the effect of Pt-Sn structural characteristics

The electrocatalytic activity for CO, H₂/CO and CH₃OH oxidation of Pt-Sn catalysts has been extensively investigated for a possible use as anode materials for low-temperature fuel cells. This paper presents an overview of the relationship between the structural characteristics of the catalysts (catalyst composition, degree of alloying, presence of oxides) and their electrocatalytic activity for the oxidation of the different fuels.     

Production of middle distillate in a dual-bed reactor from synthesis gas through wax cracking: Effect of acid property of Pd-loaded solid acid catalysts on the wax conversion and middle distillate selectivity

Although alumina-supported gold nanoparticles are poor catalysts for the oxidation of carbon monoxide, they have turned out to be promising candidates for the preferential oxidation of CO in hydrogen-rich streams (PrOx), as hydrogen apparently enhances the CO oxidation rate. The mechanism of this promotion effect is unclear. In this study, we carry out kinetic measurements on the PrOx reaction catalyzed by a 0.9% Au/Al₂O₃ catalyst, which is prepared by direct anionic exchange. We show that the apparent activation energy of the oxidation of CO is lower than that of the oxidation of H₂, whatever the hydrogen content in the feed. On the other hand, the hydrogen partial reaction order is higher in the oxidation of H₂ than in the oxidation of CO. Thus, the CO oxidation rate is significantly increased at low temperature by the introduction of only a small amount of hydrogen in the reactant mixture. At higher temperatures, the selectivity to CO₂ decreases due to competition with the oxidation of H₂. Higher hydrogen concentrations cause the competition between CO and H₂ oxidations to start at lower temperatures. It is proposed that hydrogen reacts with oxygen to yield highly oxidizing intermediates that selectively react with CO as long as the energetic barrier to produce water from these intermediates is not crossed.     

富氢气体中CO优先氧化催化剂研究进展

介绍了富氢气体中CO优先氧化贵金属与非贵金属催化剂的研究进展,包括Pt催化剂、Au催化剂和Cu催化剂等,综述了载体、助剂、预处理和制备方法等对优先氧化性能的影响,重点评述改善Pt催化剂低温活性的方法.     

A comparison of CO oxidation on ceria-supported Pt,Pd, and Rh

Steady-state, CO-oxidation kinetics have been studied at differential conversions on model, ceria-supported, Pt, Pd, and Rh catalysts, from 467 to 573 K, and the results compared to the alumina-supported metals. On each of the ceria-supported metals, there is a second mechanism for CO oxidation under reducing conditions which involves oxygen from ceria reacting with CO on the metals. The rates of this second process are independent of which metal is used. The process has a significantly lower activation energy (14±1 kJ/mol compared to 26±2 kJ/mol on alumina-supported catalysts) and different reaction orders for both CO (zeroth-order compared to –1) and 02 (0.40 to 0.46 compared to first-order). This second process leads to significant rate enhancements over alumina-supported catalysts at low temperatures, especially for Pt. The implications of these results for automotive catalysis are discussed.     

Characteristics of adsorbed CO and CH3OH oxidation reactions for complex Pt/Ru catalyst systems

Pt/Ru powder catalysts of the same nominal Pt to Ru composition were prepared using a range of methods resulting in different catalyst properties. Two PtRu alloy catalysts were prepared, one of which has essentially the same surface and bulk Pt to Ru composition, while the second catalyst is surface enriched with Ru. Two powders consisting of non-alloyed Pt phases and surfaces enriched with Ru were also prepared. The oxidation state of the surface Ru of the latter two catalysts is mainly metallic Ru or Ru-oxides. The catalyst consisting of Ru-oxides was formed at 500 °C. Part of this catalyst was then reduced in a H₂ atmosphere under “mild” conditions, thus catalyst properties such as particle size are not changed, as they are locked in during previous high temperature treatment. The oxidation kinetics of adsorbed CO (CO_ads) and solution CH₃OH were studied and compared to the Ru ad-metal state and Pt to Ru site distribution of the as-prepared catalysts. The kinetics of the CO_ads oxidation reaction were observed to be slower for the catalyst containing Ru-oxides as opposed to mainly Ru metal. The CH₃OH oxidation activities measured per Pt surface area, i.e., the catalytic activities are better (by ca. seven times) for the alloy catalysts than the non-alloyed Pt/Ru catalysts. The latter two catalysts showed essentially the same catalytic CH₃OH oxidation activities, i.e., independent of the Ru ad-metal oxidation state of the as-prepared catalysts. Furthermore, it is shown that CO_ads oxidation experiments can be used to extract characteristics that allow the comparison of catalytic activities for the CO_ads oxidation reaction and Pt to Ru site distribution for complex catalyst systems.     

Promotional effect of hydroxyl on the aqueous phase oxidation of carbon monoxide and glycerol over supported Au catalysts

Gold particles supported on carbon and titania were explored as catalysts for oxidation of CO or glycerol by O₂ at room temperature in liquid-phase water. Although Au/carbon catalysts were not active for vapor phase CO oxidation at room temperature, a turnover frequency of 5 s⁻¹ could be achieved with comparable CO concentration in aqueous solution containing 1 M NaOH. The turnover frequency on Au/carbon was a strong function of pH, decreasing by about a factor of 50 when the pH decreased from 14 to 0.3. Evidently, a catalytic oxidation route that was not available in the vapor phase is enabled by operation in the liquid water at high pH. Since Au/titania is active for vapor phase CO oxidation, the role of water, and therefore hydroxyl concentration, is not as significant as that for Au/carbon. Hydrogen peroxide is also produced during CO oxidation over Au in liquid water and increasing the hydroxyl concentration enhances its formation rate. For glycerol oxidation to glyceric acid (C₃) and glycolic acid (C₂) with O₂ (1–10 atm) at 308–333 K over supported Au particles, high pH is required for catalysis to occur. Similar to CO oxidation in liquid water, H₂O₂ is also produced during glycerol oxidation at high pH. The formation of the C-C cleavage product glycolic acid is attributed to peroxide in the reaction.     

CO氧化催化剂研究进展

CO氧化反应是控制汽车尾气污染和理论研究的重要课题之一。贵金属催化剂活性好，寿命长，但由于资源缺乏，价格上涨，人们一直研究用非贵金属或少量贵金属来替代贵金属催化剂。稀土在CO氧化催化剂中的作用越来越引起人们的重视。介绍了贵金属、非贵金属和稀土催化剂。     

Oxidation of CO over Ru containing perovskite type oxides

Perovskite type catalysts La_0.7Sr_0.3Cr_1−xRu_xO₃ (0.025 ≤ x ≤ 0.100) were synthesized by annealing a mixture of metal oxides and carbonates gradually up to 1000 °C in air, and characterized by XRPD, XPS, TPD, SEM-EDS and the van der Pauw method. The CO oxidation activity was investigated in a differential recycle reactor. According to the XRPD results, all samples achieved a perovskite structure, with a small presence of SrCrO₄ phase. The XPS results revealed that the surface composition of all samples differed considerably from the stoichiometric value with an important segregation of strontium and mainly ruthenium with regard to chromium at the surface of the catalysts. The sharp decrease of resistivity with increasing surface concentration of ruthenium and the independence of the resistivity on temperature for the sample with x = 0.100 imply the possible presence of SrRuO₃, La–Ru–O and highly dispersed RuO₂ (invisible by XRPD), known as good electric conductors, at the surface. The CO oxidation activity increases with increasing the degree of substitution (x). The surface concentrations of ruthenium are almost the same in the samples with x = 0.075 and 0.100. Those samples showed the similar values of resistivity in whole investigated temperature range and very close CO oxidation activity, which indicates that the concentration of Ru⁴⁺ in the surface region and its stability are determining factors for the CO oxidation activity. The main results of this study are that ruthenium perovskites have a high thermal stability and CO oxidation activity.