Enhanced carbon monoxide oxidation activity over gold–ceria nanocomposites

The composites with nanosized gold loaded in nanoporous ceria were prepared on a large scale by a facial and environment-benign sol–gel process. These materials were characterized by XRD, ICP, BET, UV–vis absorption, XPS, HRTEM and EDX. The main factors such as size and surface state of the Au and defect in the ceria could be adjusted by acid treatment of the composites in ascorbic acid solution to improve the activity and decrease the content of noble metal Au in the materials. Enhanced catalytic activities were obtained for the CO oxidation reaction over the catalysts due to the small crystal sizes with narrow size distributions of gold nanoparticles, a large amount of defects in the nanoporous ceria support, as well as a high ratio of Au³⁺/Au⁰ in the nanocomposites.     

Gold catalysts on Co-doped ceria for complete benzene oxidation: Relationship between reducibility and catalytic activity

Very high catalytic activity in complete benzene oxidation (CBO) was observed over gold catalyst on prepared by mechanochemical mixing CeO₂–Co₃O₄ (10 wt.% of dopant). It was significantly higher compared with gold catalysts supported on ceria doped with 5 wt.% or 15 wt.% Co₃O₄. The presence of Co₃O₄ phase and Co-modified ceria was observed by XRD data. The HRTEM/HAADF results revealed that the doping with 10 wt.% Co₃O₄ was favorable for the higher gold dispersion. The highest reducibility, i. e. ability of oxygen supplying of gold catalyst on ceria doped with 10 wt.% Co₃O₄ correlates with the highest oxidation activity in CBO.     

Enhanced carbon monoxide oxidation activity over gold–ceria nanocomposites

The composites with nanosized gold loaded in nanoporous ceria were prepared on a large scale by a facial and environment-benign sol–gel process. These materials were characterized by XRD, ICP, BET, UV–vis absorption, XPS, HRTEM and EDX. The main factors such as size and surface state of the Au and defect in the ceria could be adjusted by acid treatment of the composites in ascorbic acid solution to improve the activity and decrease the content of noble metal Au in the materials. Enhanced catalytic activities were obtained for the CO oxidation reaction over the catalysts due to the small crystal sizes with narrow size distributions of gold nanoparticles, a large amount of defects in the nanoporous ceria support, as well as a high ratio of Au³⁺/Au⁰ in the nanocomposites.     

Effects of incorporation of ions into Au/TiO2 catalysts for carbon monoxide oxidation

A number of anions and cations have been incorporated into TiO₂ as support for gold catalysts and also into as-prepared Au/TiO₂ catalysts at levels of 0.4 mol% and 2.5 mol% with respect to the support. The activities of the catalysts for CO oxidation reveal that the at the higher concentration level of the ions, in all cases, a decrease in activity compared with unmodified Au/TiO₂. However, and more interestingly, addition of only 0.4 mol% of the ions to the support, prior to gold addition, in most cases resulted in activity enhancement whilst similar addition to Au/TiO₂ resulted in decrease in activity. Attempts have been made to understand the origin of these effects.     

Influence of the support on the catalytic properties of nickel/ceria in carbon monoxide and benzene hydrogenation

In this work the catalytic properties of nickel supported on various supports (Al₂O₃, SiO₂, CeO₂) in syngas conversion are compared. The influence of the temperature of reduction pretreatment was studied. The characterization of the catalysts was performed by temperature programmed reduction, isothermal reduction, CO and H₂ chemisorption, X-ray diffraction, X-ray absorption spectroscopy, magnetization and X-ray photoelectron spectroscopy. The modification of the catalytic properties of Ni/CeO₂ catalysts with reduction pretreatment is correlated to the transformation of the CeO₂ support and to strong interactions between these species and metal particles.     

Sialon-supported catalysts for deep oxidation of carbon monoxide and hydrocarbons

Catalysts supported on a new class of supports—β-Sialons Si_{6 ? z} Al _z O _z N_{8 ? z} (z = 1, 3, 4) modified with transition-metal oxides—were synthesized and tested in the complete and partial oxidation of hydrocarbons and CO. These Sialons were prepared by self-propagating high-temperature synthesis (SHS). Catalytic activity in the test reactions appeared in systems that contain unary, binary, or ternary mixtures of metal oxides of the second half of the 3d row. Cobalt oxide-based catalysts showed the highest activity. Methods were proposed for activating Sialon supports with a considerable enhancement of the activity of a supported catalyst. More precise investigative tools are required for refining new data and understanding the nature and operation mechanism of Sialons. On account of their high thermal stability and wear resistance, Sialon-supported catalysts are proposed for use in deep oxidation processes in abrasive and acid solutions and at elevated temperatures.     

Preparation and characteristics of pretreated Pt/alumina catalysts for the preferential oxidation of carbon monoxide

The polymer electrolyte membrane fuel cell (PEMFC) needs purified hydrogen fuel from hydrocarbon reforming and water-gas shift (WGS) reaction. Concentration of CO should be 10 ppm level to avoid poisoning of the platinum anode electrode. For this, preferential oxidation of carbon monoxide (PROX) reaction is essential. In this study, a novel pretreatment technique was applied to a conventional Pt/γ-Al₂O₃ catalyst. Oxygen-treated, water-treated, and conventional Pt/γ-Al₂O₃ catalyst were prepared and their performances in the PROX reaction were investigated in a simulated hydrogen-rich reaction conditions. Our results showed that catalytic activity of the oxygen-treated 5% Pt/γ-Al₂O₃ catalyst for the CO conversion increased dramatically especially at the low temperature below 100 °C. The enhancement is attributed to the formation of well-dispersed small Pt particles.     

Low-temperature prepared highly effective ferric hydroxide supported gold catalysts for carbon monoxide selective oxidation in the presence of hydrogen

Ferric hydroxide supported Au catalysts prepared with co-precipitation method at room temperature without any heat treatment hereafter exhibited high catalytic activity and selectivity for CO oxidation in air and CO selective oxidation in the presence of H₂. With calcination temperature rising, both activity and selectivity decreased. X-ray Photoelectron Spectra (XPS) indicated that Au existed as Au⁰ and Au⁺ in the catalyst without heat treatment and even after being calcined at 200 °C, while after being calcined at 400 °C, Au existed as Au⁰ completely. X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopic (HRTEM) investigations indicated that both the supports and Au species were highly dispersed as nano or sub-nano particles even after being calcined at 200 °C, but after being calcined at 400 °C the supports transformed to crystal Fe₂O₃ with typical diameter of 30 nm and Au species aggregated to nano-particles with typical diameter of 2–4 nm. HRTEM investigations also suggested that the supports calcined at 200 °C were composed of amorphous ferric hydroxide and crystal ferric oxide. Results of computer simulation (CS) showed that O₂ was adsorbed on Au crystal cell and then were activated, which should be the key factor for the subsequent reaction. It also suggested that O₂ species were more easily adsorbed on Au⁺ than on Au⁰, indicating that higher positive charge of the Au species possessed the higher activity for CO oxidation.     

催化CO氧化的铜系催化剂的研究进展

综述了近年来铜催化剂体系催化CO氧化反应的研究进展。阐述了催化剂的制备方法、制备条件及载体种类等对催化剂催化活性及稳定性的影响。通过加入添加剂、合适的制备方法和处理条件等手段是提高催化剂活性和选择性有效的途径。     

SHS-produced intermetallides as catalysts for deep oxidation of carbon monoxide and hydrocarbons

Investigated was the catalytic activity of multicomponent metal catalysts prepared by alkaline leaching of SHS-produced (NiAl₃) _x (CoAl₃) _y and (NiAl₃) _x (CoAl₃) _y (MnAl₃) _z intermetallides in deep oxidation of carbon monoxide and hydrocarbons. The morphology and composition of starting and leached intermetallides were characterized by SEM, while the specific surface of the catalysts was determined by BET method. The above preparation technique can be expected to open up a new route to synthesis of high-efficiency polymetallic catalysts for deep oxidation of CO and hydrocarbons.     

Kinetics of the catalytic oxidation of carbon monoxide over oxide catalysts

The kinetics of the oxidation of carbon monoxide over five commercially available oxide catalysts have been determined at CO concentrations of 4–15 mol% and initial temperatures up to 170°C. The reaction showed a fractional dependence on the concentration of CO and was inhibited by CO₂. From an analysis of the data a Langmuir-Hinshelwood reaction scheme is proposed.     

Kinetics of the catalytic oxidation of carbon monoxide over oxide catalysts

The kinetics of the oxidation of carbon monoxide over five commercially available oxide catalysts have been determined at CO concentrations of 4–15 mol% and initial temperatures up to 170°C. The reaction showed a fractional dependence on the concentration of CO and was inhibited by CO₂. From an analysis of the data a Langmuir-Hinshelwood reaction scheme is proposed.     

Evaluation of carbon monoxide oxidation over CeO2/Co3O4 catalysts: Effect of ceria loading

Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, S_BET = 100 m² g⁻¹) with different loading of ceria was proceeded using the impregnation method (assigned as CeX/R230, X = 4, 12, 20, 35 and 50 wt%). The CeX/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at −196 °C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area (S_BET = 109 m² g⁻¹) of cobaltic oxide, further weaken the bond strength of CoO and lower the activation of CO oxidation among CeX/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T₅₀ (temperature for 50% CO conversion) at 88 °C.     

Catalytic activity of gold supported on ZnO tetrapods for the preferential oxidation of carbon monoxide under hydrogen rich conditions

It is reported that 3 nm gold nanoparticles deposited on ZnO tetrapods show high activity for the selective oxidation of carbon monoxide in hydrogen rich streams; the catalytic activity of this system is at least twice as high as the values hither to observed on any conventional support for this reaction.     

Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts

Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the oxidation of carbon monoxide with oxygen. Mixed copper-chromium oxide active phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activity shows a maximum around C/M=100−50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium ratio of 2/1. The apparent activation energy for the carbon monoxide oxidation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu---O bond rupture is the rate-limiting process. The reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably rate determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Oxygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Therefore, the reduction of nitric oxide takes place only when all oxygen has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. Nitric oxide and molecular oxygen react preferentially with carbon monoxide, so, in an excess of oxidizing component, gasification of the carbon support occurs at higher temperatures after carbon monoxide has been completely consumed.     

Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

An investigation of the activity of coprecipitated gold catalysts for methane oxidation

Coprecipitated Au on transition metal oxide catalysts have been tested for their activity toward methane oxidation. Catalyst activities fall in the order Au/Co₃O₄>Au/NiO> Au/MnO_x> Au/Fe₂O₃ > Au/CeO. The Au/Co₃O₄ catalyst is active just below about 250°C. The catalysts are proposed to have more than one type of reactive site since the supports are also active at higher temperatures. Analysis of spent catalysts with X-ray photoelectron spectroscopy indicates that Au exists in at least two oxidation states on some of them, a reduced state and an oxidized state. The activity for methane oxidation increases with increasing oxidation of Auin the oxidized state.     

Oxidation of mono- and polyalcohols with gold: Comparison of carbon and ceria supported catalysts

Aim of this work is the investigation of the gold catalysed liquid-phase oxidation of polyalcohols with both, primary and secondary alcohol groups, in order to evaluate the potential of gold catalysts to oxidise a secondary alcohol group in presence of a primary group. For that purpose we investigated the heterogeneously catalysed liquid-phase oxidations of n-propanol, as reference for the oxidation of the primary alcohol group only, of propylene glycol, where the competitive reaction can be examined and finally of glycerol, which is the target reaction due to known economic aspects. We report on the performance of ceria supported gold catalysts in these reactions and discuss the results in dependency of the specific surface area of the support, the catalyst and support preparation method and the catalyst activation conditions. Finally, in order to estimate the relative activity and selectivity of the ceria supported catalysts we compared the catalytic results with the performance of carbon and titania supported gold catalysts.     

H2O-tolerant monolithic catalysts for preferential oxidation of carbon monoxide in the presence of hydrogen

Pt–Fe/mordenite (4 wt% Pt–0.5 wt% Fe) powder catalysts were wash-coated onto ceramic straight-channel monoliths by using silica- and/or alumina-sol as a binder, and were evaluated for the preferential oxidation of carbon monoxide (PROX) in a hydrogen-rich gas. In a synthetic reformate gas (1% CO, 1% O₂, 5% H₂O, 20% CO₂, and balance H₂), the CO concentration was reduced to less than 20 ppm at temperatures ranging from 100 to 130 °C. After a certain period of the PROX reaction, condensation of H₂O in the pores of the mordenite-support occurred over the monolithic catalyst, which was wash-coated with alumina-sol, in the lower temperature range (100–120 °C), resulting in a rapid increase in CO concentration. The monolithic catalyst wash-coated with silica-sol, however, showed an excellent tolerance against H₂O condensation and offered a stable catalytic performance, maintaining a CO concentration of ca. 20 ppm for 200 h. The H₂O-tolerant characteristic was attributed to the relatively small adsorption amount of H₂O over the silica-modified monolithic catalyst.