Promoting Role of Fe in the Preferential Oxidation of CO Over Ir/Al2O3

A novel catalyst Ir–FeO _x /Al₂O₃ designed for the preferential oxidation (PROX) of CO under excess hydrogen was developed in this work. To clarify the promoting role of Fe species, three different impregnation sequences were employed, and the resultant catalysts were characterized by various techniques. The results showed that the Ir–FeO _x /Al₂O₃ catalyst, which was prepared by the co-impregnation procedure, exhibited the best performance for the PROX. The partially exposed and highly dispersed Ir sites and the FeO _x sites allowed good adsorption for both CO and O₂ over the Ir–FeO _x /Al₂O₃, which was believed to be responsible for the enhanced activity.     

Catalytic Combustion of Benzene over Au Supported on Ceria and Vanadia Promoted Ceria

Complete oxidation of benzene over Au/CeO₂ and Au/V₂O₅/CeO₂ catalysts were studied. Gold was supported on CeO₂ from different sources by deposition precipitation method. The catalysts were characterized by XRD, BET, X-ray photoelectron spectroscopy, TEM, and H₂-TPR techniques. The catalytic activity toward the complete oxidation of benzene to CO₂ and water were strongly dependent on the kind of CeO₂ sources, loading amount of gold, modified amount of vanadia, and calcination temperature. High activities were obtained on 1% Au/CeO₂ and 2% vanadia modified Au/CeO₂ catalysts calcined at 300 °C. The nanometer size of Au particle and interaction between Au, V₂O₅, and CeO₂ play important roles in determining the activity of benzene complete oxidation.     

焙烧温度对CuO/Co_3O_4-CeO_2催化剂CO优先氧化性能的影响

采用浸渍法制备了Cu O/Co3O4-Ce O2(Cu Co Ce10)催化剂,考察了在不同温度(250、300、350、400和450℃)焙烧后的样品在富氢气氛中对CO优先氧化反应的催化性能。应用BET、XRD、H2-TPR及XAFS技术详细表征了催化剂的结构与性能。结果表明,不同温度焙烧的催化剂中,铜物种均主要以Cu O相存在;350℃焙烧的Cu Co Ce10催化剂具有最大的比表面积和最好的氧化还原性能,在98~173℃的温度范围内能够将CO完全转化为CO2,呈现出最宽的CO优先氧化可操作温度窗口,以及良好的催化活性稳定性。     

Influence of the calcination temperature on the Au/FeOx/Al2O3 catalyst

The effect of calcination temperature on the electronic structure of gold catalyst was investigated in this paper. CO oxidation was employed as a test reaction to correlate the phase changes to the catalytic activity. The TPR, XPS and XRD characterization showed that the highly active Au/FeO_x/Al₂O₃ catalyst contained both gold atoms and ions. The catalytic performance of gold catalyst may depend on the Au⁰/Au³⁺ ratio. The best activity in the present work was obtained when the Au⁰/Au³⁺ ratio was about 0.89. Copyright © 2006 Society of Chemical Industry     

Effect of Support Modification on Reduction and CO Oxidation Activity of Doped Ceria-Supported Copper Oxide Catalyst

Ceria materials were modified by doping with gadolinia or yttria and by a hold period at 260 °C for 2 h during temperature-programmed calcinations to 650 °C. These doped ceria-supported copper oxide catalysts and the doped ceria material were characterized by temperature-programmed reduction, electron paramagnetic resonance, and CO oxidation activity test. It was observed that, as the doping concentration of gadolinia increases, the reduction temperature of the copper oxide species increases and the CO oxidation activity decreases. This is due to increased formation of the surface spinel species of copper oxide with gadolinia. As the yttria content increases to greater than 10 mol%, surface segregation occurs, which causes the amount of surface oxygen vacancies to decrease. It was also found that maintaining the temperature at 260 °C during calcination may decrease the amount of oxygen vacancies. The surface oxygen vacancies may be the active sites for CO oxidation over the oxygen ion conducting materials in the absence of any metal present. Gd doping leads to the formation of extrinsic oxygen vacancies, which increases the oxygen ionic conductivity of the doped ceria and thus increases the CO oxidation activities of the supported catalysts as well as of the doped ceria.     

Activity, Selectivity, and Long-Term Stability of Different Metal Oxide Supported Gold Catalysts for the Preferential CO Oxidation in H2-Rich Gas

A comparative study of the catalytic performance and long-term stability of various metal oxide supported gold catalysts during preferential CO oxidation at 80°C in a H₂-containing atmosphere (PROX) reveals significant support effects. Compared to Au/-Al₂O₃, where the support is believed to behave neutrally in the reaction process, catalysts supported on reducible transition metal oxides, such as Fe₂O₃, CeO₂, or TiO₂, exhibit a CO oxidation activity of up to one magnitude higher at comparable gold particle sizes. The selectivity is also found to strongly depend on the employed metal oxide, amounting, e.g., up to 75% for Au/Co₃O₄ and down to 35% over Au/SnO₂. The deactivation, which is observed for all samples with increasing time on stream, except for Au/-Al₂O₃, is related to the build-up of surface carbonate species. The long-term stability of the investigated catalysts in simulated methanol reformate depends crucially on the ability to form such by-products, with magnesia and Co₃O₄ supported catalysts being most negatively affected. Overall, Au/CeO₂ and, in particular, Au/-Fe₂O₃ represent the best compromise under the applied reaction conditions, especially due to the superior activity and the easily reversible deactivation of the latter catalyst.     

Effect of calcination temperature on the catalytic activity of Au colloids mechanically mixed with TiO2 powder for CO oxidation

In order to elucidate the role of the contact structure between gold and metal oxide support in low-temperature CO oxidation, a mechanical mixture of colloidal gold with TiO₂ powder was prepared and calcined at different temperatures. The sample calcined at 473 K, which is composed of spherical gold particles with a mean diameter of 5.1 nm and TiO₂ powder, is poorly active for CO oxidation at temperatures up to 473 K. The catalytic activity appreciably increases with an increase in calcination temperature up to 873 K even though gold particles grow to larger ones, reaching a level with almost the same turnover frequency as that of Au/TiO₂ prepared by a deposition–precipitation method. This revised version was published online in July 2006 with corrections to the Cover Date.     

Density Functional Study of the CO Oxidation on a Doped Rutile TiO2(110): Effect of Ionic Au in Catalysis

We used density functional theory to examine whether doping oxides makes them better oxidation catalysts. We studied in detail titania doped with Au and used CO oxidation as a test of the oxidizing power of the system. We show that doping with Au, Ag, Cu, Pt, Pd, Ni reduces dramatically the bond of surface oxygen to titania or ceria, making them better oxidation catalysts. These calculations suggest that it is worthwhile to explore doped oxides as oxidation catalysts.     

二氧化铈载金催化剂催化CO氧化的研究进展

将金纳米颗粒负载在二氧化铈上,形成负载型催化剂,能应用于多种反应体系,表现出良好的催化活性,有着广阔的发展前景。综述了二氧化铈载金催化剂在一氧化碳催化氧化中的应用研究进展,介绍了二氧化铈载金催化剂中金的结构、价态和纳米尺寸,载体二氧化铈的形貌、结构、尺寸和氧空位以及金和载体二氧化铈两者之间的相互作用对一氧化碳催化氧化性能的影响。     

Reaction and Surface Characterization Studies of Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts for CO Preferential Oxidation in Reformed Gas

In order to investigate the reasons the activation of a Ru/Al₂O₃ catalyst by heating in a H₂/N₂ mixed gas improves the CO preferential oxidation (PROX) activity, the oxidation state of the Ru on the catalyst surface was studied by using ESCA. As the ratio of Ru(0) to total Ru on the surface was increased, the temperature window of the Ru catalyst, where CO was reduced to below 10 ppm, was expanded to the lower temperature side. The activity of CO oxidation by O₂ of the Ru catalyst at lower temperatures was improved by increasing the ratio of Ru(0). However, the selectivity for CO oxidation hardly varied with the change in the surface Ru(0) ratio at these low temperatures. It is considered that O₂ activation on Ru(0) plays an essential role in CO PROX activity on the Ru catalyst at low temperatures.     

ZnO/SBA-16不同焙烧温度对CuO/ZnO/SBA-16催化CO活性的影响

通过浸渍法将氧化锌负载与载体SBA-16上,制得不同焙烧温度的ZnO/SBA-16,再通过沉淀沉积法将氧化铜负载与ZnO/SBA-16上,制备出高分散的催化剂10%CuO/6%ZnO/SBA-16并考察了其对CO氧化的活性,通过XRD、TG-DTA、H2-TPR表征及活性测试,发现300℃、400℃、500℃预先焙烧过的ZnO/SBA-16制得的催化剂中CuO分散度更好,活性更高。     

Preferential oxidation of carbon monoxide over Pt, Au monometallic catalyst, and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate

The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO₂ catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO₂ catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO₂ and H₂O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.     

The Role of Impurities on the Low Temperature CO Oxidation on Au/TiO2

A number of Au/TiO2 catalysts have been prepared by a variety of methods in order to assess the affect of catalysts preparation methods on performance, catalyst contamination and the interplay between them. Their activity was studied in a pulse flow microreactor and it was found that preparation methods designed to eliminate impurities lead to more active samples. The effect of chlorine is often cited to be detrimental in the literature, but we have quantified it using XPS. It was found that the activity decreased in a nearly linear fashion with an increasing amount of this element at the surface. It is generally considered that catalysts prepared by the incipient wetness (IW) technique are ineffective for CO oxidation, but we show here that, by appropriate preparation methods, high activity IW catalysts can be made.     

制备参数对Au/TiO_2催化剂上CO低温氧化性能的影响

以溶胶-凝胶法制备TiO2载体,用沉积-沉淀法制备出一系列负载型Au/TiO2。系统考察了焙烧温度、金的负载量、反应液pH值、沉淀剂种类以及Cl-存在与否等制备参数对催化剂活性的影响。以室温下CO的催化氧化为探针反应,确定催化剂的最适宜制备参数,并对优化的质量分数为1.0%的Au/TiO2催化剂进行了活性稳定性测试。结果表明：Au/TiO2的最适宜焙烧温度是200～350℃;反应液的最适宜pH值为9;最适宜沉淀剂是NaOH;金的负载量（质量分数,下同）在0.5%～5.0%范围内时,金含量越高,催化剂活性和热稳定性越好。大量Cl-的存在能导致催化剂活性的显著下降。对优化的Au/TiO2催化剂在室温下催化氧化不同浓度的CO进行循环测试,经历3次循环,连续反应2 160 min后,CO的转化率仍为100%。     

The oxidation and scrambling of CO with oxygen at room temperature on Au/ZnO

An FTIR and quadrupole mass study of CO adsorption and oxidation with¹⁶O₂ and¹⁸O₂ on Au/ZnO catalysts is presented. The experimental results indicate that: (i) CO is activated by gold in two molecular forms, a linear carbonyl species bonded at terrace Au sites and a carbonyl species bonded to Au peripheral sites; (ii) a band related to CO adsorbed on Au oxidized sites and a scrambling reaction between CO and¹⁸O₂ indicate that oxygen is also activated on gold sites. The oxygen adsorbed on gold is probably strongly basic, as is the oxygen adsorbed on silver and on copper, and it can easily oxidize CO to CO₂.     

Supported base metal catalysts for the preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX)

Supported base metal catalysts were tested for the preferential oxidation of CO (CO PROX). The catalysts we investigated covered a wide range of transition metals (Co, Cr, Cu, Ni, Zn) supported on oxides with very different acidic, basic and redox properties (MgO, La₂O₃, SiO₂–Al₂O₃, CeO₂, Ce_0.63Zr_0.37O₂). The influence of the metal loading (Cu), the support properties (acidity, basicity, redox, surface area) and the reaction conditions (reaction temperature, feed composition) on the catalyst activity and selectivity was evaluated. The activity of ceria and ceria–zirconia supported copper catalysts was comparable to the performances of noble metal samples classically used for the PROX reaction. In addition, Cu–CeO₂ catalysts showed a practically constant and high selectivity towards CO oxidation in the temperature range of 50–150 °C. Due to the strong synergetic effect between copper and ceria, only a small amount of copper (0.3 wt.%) was necessary to get an active catalyst. The best catalytic performances were obtained for the samples containing 1–3 wt.% copper. The presence of small copper particles in close interaction with the ceria support was shown to be responsible for the enhanced activity. Except for the hydrogen oxidation, no parallel reactions (CO or CO₂ methanation reactions, coking, RWGS) could be detected over these catalysts. Classically, an increase of the oxygen excess led to an increased CO conversion with a simultaneous loss of selectivity towards CO₂. Finally, the presence of CO₂ in the feed negatively affected the catalytic activity. This effect was attributed to the adsorption of CO₂ on the copper sites, probably as CO.     

ZrO_2负载Cu催化剂对CO低温催化氧化研究

用溶胶-凝胶-超临界干燥法制备了纳米氧化锆。采用沉淀法制备氧化锆负载铜催化剂。制备的催化剂用X-ray射线衍射(XRD),透射电镜(TEM),比表面积(BET)和H2-TPR等进行了表征。研究了催化剂的焙烧温度和负载比例对CO转化效率的影响,其最佳焙烧温度为650℃,Zr与Cu的最佳物质的量比是10:8。获得催化剂在温度为68℃具有催化活性,176℃时CO的转化率达到50%,较好地实现了ZrO2负载Cu在较低温度下对CO的催化。     

Pd/Al2O3催化剂上低浓度CO氧化的研究

采用共沉淀法和浸渍法制备了2%Pd-Al2O3催化剂,在固定反应器中评价不同制备方法(共沉淀和浸渍)、助剂(1%K和1%Mg)、预处理方法(快速氧化和缓慢还原)、空速(726h-1、1638h-1和5274h-1)情况下2%Pd-Al2O3催化剂对CO氧化的催化活性。结果表明,共沉淀法制备的2%Pd50%Zr-Al2O3催化剂有较高的反应活性,该催化剂经缓慢还原预处理活化,在温度为70℃时就可以使低浓度的CO完全氧化。     

Macro‐ and Microkinetic Simulation of Diesel Oxidation Catalyst: Effect of Aging,Noble Metal Loading and Platinum Oxidation

In automotive exhaust aftertreatment simulation, both macro‐ and microkinetic models are commonly used. In this contribution both models are applied for the simulation of diesel oxidation catalysts (Pt/γ‐Al₂O₃) with different catalyst loading and degree of thermal aging. The study proves that the structure insensitive kinetics of the considered catalysts can be described with the same rate equations only by scaling the rate constants of the different reaction steps with the catalytically active Pt surface, which is accessible by CO adsorption or light‐off measurements. In addition, NO oxidation is strongly influenced by a reversible, slow transformation of Pt into Pt‐oxide.     

Au/ZnO and Au/Fe2O3 catalysts for CO oxidation at ambient temperature: comments on the effect of synthesis conditions on the preparation of high activity catalysts prepared by coprecipitation

The preparation of Au/ZnO and Au/Fe₂O₃ catalysts using two coprecipitation methods is investigated to determine the important factors that control the synthesis of high activity catalysts for the oxidation of carbon monoxide at ambient temperature. In particular, the factors involved in the preparation of catalysts that are active without the need for a calcination step are evaluated. The two preparation methods differ in the manner in which the pH is controlled during the precipitation, either constant pH throughout or variable pH in which the pH is raised from an initial low value to a defined end point. Non-calcined Au/ZnO catalysts prepared using both methods are very sensitive to pH and ageing time, and catalysts prepared at a maximum pH = 5 with a short ageing time (ca. 0–3 h) exhibit high activity. Catalysts prepared at higher pH give lower activity. However, all catalysts require a short operation period during which the oxidation activity increases. In contrast, the calcined catalysts are not particularly sensitive to the preparation conditions. Non-calcined Au/Fe₂O₃ catalysts exhibit high activity when prepared at pH ≥ 5. Calcined Au/Fe₂O₃ prepared using the controlled pH method retain high activity, whereas calcined catalysts prepared using the variable pH method are inactive. The study shows the immense sensitivity of the catalyst performance to the preparation methods. It is therefore not surprising that marked differences in the performance of supported Au catalysts for CO oxidation that are apparent in the extensive literature on this subject, particularly the effect of calcination, can be expected if the preparation parameters are not carefully controlled and reported.