Comparison of the activity of Au/CeO2 and Au/Fe2O3 catalysts for the CO oxidation and the water-gas shift reactions

We compare the activity and relevant gold species of nanostructured gold–cerium oxide and gold–iron oxide catalysts for the CO oxidation by dioxygen and water. Well dispersed gold nanoparticles in reduced form provide the active sites for the CO oxidation reaction on both oxide supports. On the other hand, oxidized gold species, strongly bound on the support catalyze the water-gas shift reaction. Gold species weakly bound to ceria (doped with lanthana) or iron oxide can be removed by sodium cyanide at pH ≥12. Both parent and leached catalysts were investigated. The activity of the leached gold–iron oxide catalyst in CO oxidation is approximately two orders of magnitude lower than that of the parent material. However, after exposure to H₂ up to 400 °C gold diffuses out and is in reduced form on the surface, a process accompanied by a dramatic enhancement of the CO oxidation activity. Similar results were found with the gold–ceria catalysts. On the other hand, pre-reduction of the calcined leached catalyst samples did not promote their water-gas shift activity. UV–Vis, XANES and XPS were used to probe the oxidation state of the catalysts after various treatments.     

Study on catalytic performance of CeO2/CuO catalysts for preferential CO oxidation

采用水热法合成纳米尺寸的CuO,然后采用微乳液法或浸渍法将CeO₂负载在CuO上制备逆负载的CeO₂/CuO催化剂。通过X射线衍射（XRD）、程序升温还原（TPR）、比表面分析（BET）和富氢气中CO优先氧化活性测试等研究手段对催化剂进行了表征。研究发现,CeO₂/CuO催化剂的活性和选择性与CeO₂和CuO颗粒的尺寸密切相关,大颗粒的CuO载体有利于提高催化剂的选择性;小颗粒的氧化铈负载在大颗粒的氧化铜上,可以产生更多两相接触界面,有助于提高催化剂的活性。     

Spectroscopic features and reactivity of CO adsorbed on different Au/CeO2 catalysts

An FTIR study of CO adsorption from 120 K up to room temperature on a series of Au–ceria samples is presented. Samples with low gold content (0.7 and 0.6 at%) were prepared by urea gelation/co-precipitation and by cyanide leaching of the high-gold content (5.8 at%) material prepared by deposition–precipitation on La-doped CeO₂. The samples were subjected to different pretreatments to collect information on the surface composition under working conditions. An absorption band at 2130–2140 cm⁻¹, not reversible on outgassing and more resistant to oxidation than the usual carbonyl band on Au⁰ sites, was present due to CO adsorbed on cationic gold clusters. This highly stable species is relevant for hydrogen gas upgrade by removing CO from reformate-type gases at low temperatures. In addition, a broad absorption band in the 2000–2100 cm⁻¹ range was observed after reduction in hydrogen, due to structural and electronic changes of gold. Interestingly, the reduced gold species in ceria can be reoxidized at mild conditions. Light-off of the CO oxidation reaction took place below room temperature on the metallic gold-containing ceria but was delayed until 310 K on the ionic gold-containing sample. TPR and XPS analysis of the fresh and used catalysts corroborated the stability of ionic gold in ceria up to 393 K in the reaction gas mixture.     

The preparation of Au/CeO2 catalysts and their activities for low-temperature CO oxidation

Au/CeO₂ catalysts prepared by co-precipitation (CP) and deposition-precipitation (DP) methods were tested for low temperature CO oxidation reaction. The structural characters and redox features of the catalysts were investigated by XRD, XPS and H₂-TPR. Their catalytic performances for low temperature CO oxidation were studied by means of a microreactor -GC system. It showed that the catalytic activities of Au/CeO₂ catalysts greatly depended on the preparation method. The catalysts prepared by DP method exhibited a surprisingly higher activity towards CO oxidation than that prepared by CP method. This may arise from the differences in the particle sizes of Au and redox properties of the catalysts. The low Au loading and the resistance to high temperature of DP-prepared catalyst made it more applicable.     

Mn/Mg/Al-spinels as catalysts for SOx abatement: Influence of CeO2 incorporation and catalytic stability

Mg,Mn,Al-oxides with spinel structure, Al/(M²⁺ + Al) molar ratio of 0.25 and 0.50 and an Mn/Mg molar ratio of 0.30 have been evaluated as catalysts for SO_x removal under conditions similar to those found in FCC units. The best performance was that of the sample with the higher aluminium content. The incorporation of CeO₂ in this sample favored SO_x uptake for short reaction times as well as the reduction of the sulfated catalysts. When the regeneration was started at 530 °C, only H₂S was observed as reaction product, but when this step started at 650 °C, the release of SO₂ preceded that of H₂S, regardless of the chemical composition of the sample. As to the additive performance for successive reaction–regeneration cycles, the incorporation of CeO₂ produced a less efficient catalyst with regard to the removal of the SO₂ along the process, but with a higher regeneration efficiency and a lower formation of SO₂ as regeneration product.     

Physicochemical and catalytic properties of nanosized Au/CeO2 catalysts for eco-friendly oxidation of benzyl alcohol

In this work, benzyl alcohol oxidation was investigated over Au/CeO₂-homogeneous deposition–precipitation (ACH) and Au/CeO₂-direct anionic-exchange (ACD) catalysts. Various characterization techniques were employed to study their physicochemical properties. TEM images revealed presence of 5.3 and 7.4 nm Au nanoparticles in ACH and ACD catalysts, respectively. Raman studies showed that only ACH sample exhibits oxygen deficiency (0.0574). Amongst, the ACH catalyst exhibited better catalytic performance owing to smaller gold nanoparticles and abundant oxygen vacancies. The alcohol conversion and product selectivity were strongly dependent on temperature and time-on-stream conditions. The catalytic activity decreased after repeated use due to aggregation of Au nanoparticles.     

Effect of the preparation method on Au/Ce-Ti-O catalysts activity for VOCs oxidation

Studies concerning the preparation of gold phases dispersed on binary Ce-Ti oxide (Ce_0.3Ti_0.7O₂) were performed in order to elaborate catalysts for total oxidation of VOCs. Solids containing gold, cerium and titanium were synthesized by impregnation and deposition precipitation (DP) method using NaOH, Na₂CO₃ or urea as precipitant agent. These catalysts have been characterized by means of total surface area (BET), X-ray diffraction (XRD), diffuse reflectance ultra-violet–visible spectroscopy (DR/UV–vis) and temperature programmed reduction (TPR) and their reactivity towards the oxidation of propene was studied. Thus, it was revealed that the gold-based material prepared by DP method using urea as precipitant agent was the most efficient catalyst towards the total oxidation of propene. Based on the characterisation data, it has been shown that the preparation method has an effect on the catalytic activity.     

Nanosized CeO2-supported metal oxide catalysts for catalytic reduction of SO2 with CO as a reducing agent

Nanosized CeO₂ was synthesized by sol–gel method and used as the support to prepare six kinds of supported metal oxide catalysts by incipient wetness impregnation method. These catalysts were investigated for the catalytic reduction of SO₂ to elemental sulfur with CO as a reducing agent. Experimental results indicate that Cr₂O₃/nano-CeO₂ was the most active catalyst. Using this catalyst, a kinetic study was performed on the reduction of SO₂ and the optimal feed ratio of CO/SO₂ was found to be 2.5/1, and low concentrations of SO₂ and CO provide a high SO₂ conversion and sulfur yield. We also found that the catalyst presulfided by CO + SO₂ exhibits a higher performance than those pretreated with CO, SO₂ or He. The discrepancy in the stability and activity resulted in the pretreatment has been rationally explained. The temperature-programmed desorption patterns of SO₂ and CO illustrate that Cr₂O₃/nano-CeO₂ can adsorb and desorb SO₂ and CO more easily than can other catalysts. These results may properly explain why Cr₂O₃/nano-CeO₂ has a higher activity for the reduction of SO₂.     

负载型CeO2/OMS -2催化剂对VOCs的催化氧化性能

采用回流法合成了OMS -2,同时以OMS -2为载体,采用浸渍法负载CeO2制备了Ce/OMS-2催化剂,运用X射线衍射、拉曼光谱、BET、H2 - TPR和NH3 - TPD等方法对催化剂进行表征,并考察CeO2/OMS -2催化剂催化氧化二氯甲烷和乙酸乙酯的性能.结果表明,CeO2的加入并未改变OMS -2较好的八面体分子筛结构,CeO2的晶粒很小且以高分散的形式存在;负载CeO2的OMS -2催化剂比表面积都有增大趋势;随着CeO2负载量的增加,低温和高温的还原峰都先向低温方向偏移再向高温方向偏移;随着CeO2负载量的增加,高温处强酸性峰先向低温方向偏移再向高温方向偏移.反应活性结果表明,CeO2/OMS -2催化剂活性比载体OMS -2的活性好,随着铈负载量的增加,催化活性先升高再降低,其中,CeO2负载质量分数为1.0％(1.0Ce/OMS -2)时催化剂效果最好,这可能是由于铈的加入活化了OMS -2中的氧,影响了整体催化剂的酸性,进而影响反应性能.     

Influence of pretreatment atmospheres on the activity of Au/CeO2 catalyst for low-temperature CO oxidation

Au/CeO₂ catalyst was prepared via a deposition–precipitation method and further pretreated in different atmospheres prior to CO oxidation. A reductive atmosphere pretreatment slightly improved the low-temperature activity of Au/CeO₂. The greatest activity was achieved when Au/CeO₂ catalyst was subjected to an oxidative atmosphere followed by a reductive atmosphere pretreatment. Analysis using XRD, TEM and nitrogen sorption measurement showed that pretreatment retains the crystal phase, morphology and pore structure of CeO_2. However, there was a change in the interaction between gold species and CeO₂ support, as revealed by H₂-TPR measurement.     

Promotion effect of metal oxides on inverse CeO2/CuO catalysts for preferential oxidation of CO

The MO_x–CeO₂/CuO (M = Co, Mn, Sn and Zn) catalysts were synthesized by the hydrothermal method and characterized by XRD, BET, SEM, H₂-TPR and HRTEM techniques. It is found that the MnO₂–CeO₂/CuO catalyst exhibits the best activity from 75 °C to 115 °C, suggesting that the addition of Mn is the most effective for improving low-temperature activity. The reasons are that MnO₂ improves the dispersion of CeO₂ and the textural property of CeO₂/CuO catalyst. Moreover, the presence of MnO₂ is favorable for preventing the reduction of CuO, and MnO₂ also enhances the interaction between CeO₂ and CuO.     

Honeycomb supported Co3O4/CeO2 catalyst for CO/CH4 emissions abatement: Effect of low Pd–Pt content on the catalytic activity

A structured Co₃O₄–CeO₂ composite oxide, containing 30% by weight of Co₃O_4, has been prepared over a cordieritic honeycomb support. The bimetallic, Pd–Pt catalyst has been obtained by impregnation of the supported Co₃O₄–CeO₂ with Pd and Pt precursors in order to obtain a total metal loading of 50 g/ft³.CO, CH₄ combined oxidation tests were performed over the catalyzed monoliths in realistic conditions, namely GHSV = 100,000 h⁻¹ and reaction feed close to emission from bi-fuel vehicles. The Pd–Pt un-promoted Co₃O₄–CeO₂ is promising for cold-start application, showing massive CO conversion below 100 °C, in lean condition.A strong enhancement of the CH₄ oxidation activity, between 400 and 600 °C, has been observed by addition to the Co₃O₄–CeO₂ of a low amount of Pd–Pt metals.     

制备条件对Au/TiO2液相氧化反应活性的影响

采用沉积-沉淀法制备纳米级Au/TiO_2催化剂,以葡萄糖液相催化氧化制葡萄糖酸为探针反应,考察了催化剂制备条件对Au/TiO_2活性的影响,并利用TEM、XRD和XPS等方法对催化剂进行了表征。结果表明,Au/TiO_2对葡萄糖液相氧化反应的催化活性与催化剂的制备条件密切相关,纳米金的颗粒尺寸不是决定催化活性的惟一因素,金在催化剂中的价态对催化活性有重要影响。     

Characterization and catalytic activity of unpromoted and alkali (earth)-promoted Au/Al2O3 catalysts for low-temperature CO oxidation

Various unpromoted and alkali (earth) promoted gold catalysts were characterized by means of XRD, HRTEM, DR/UV–Vis and TPR. Based on the results we conclude that metallic Au is the active species in CO oxidation and that the reduction of Au³⁺ to Au⁰ proceeds below 200 °C. Pretreatment at mild temperatures, viz. 200 °C, results in the highest catalytic performance of Au/Al₂O₃ in low-temperature CO oxidation. Alkali (earth) metal oxide additives are most probably structural promoters. The best promoting effect is found for BaO.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Decomposition of ozone on Ag/SiO2 catalyst for abatement of waste gases emissions

A silica-supported Ag system made by the incipient wetness impregnation method was investigated in the reaction of heterogeneous catalytic decomposition of ozone. It was established that the catalytic ozone decomposition on Ag/SiO₂ proceeded in the temperature interval −40 °C to 25 °C as a first order reaction with activation energy of 65 kJ/mol (pre-exponential factor 5.0 × 10¹⁴ s⁻¹). Based on the results from the instrumental methods (SEM, XRD, XPS, EPR, TPD) it can be concluded that in presence of ozone the silver is oxidized to a complicated mixture of Ag₂O₃ and AgO. Due to the high activity and stability of the Ag/SiO₂ catalyst, it is promising for neutralization of waste gases containing ozone.     

负载型CeO_2/OMS-2催化剂对VOCs的催化氧化性能

采用回流法合成了OMS-2,同时以OMS-2为载体,采用浸渍法负载CeO_2制备了Ce/OMS-2催化剂,运用X射线衍射、拉曼光谱、BET、H_2-TPR和NH_3-TPD等方法对催化剂进行表征,并考察CeO_2/OMS-2催化剂催化氧化二氯甲烷和乙酸乙酯的性能。结果表明,CeO_2的加入并未改变OMS-2较好的八面体分子筛结构,CeO_2的晶粒很小且以高分散的形式存在;负载CeO_2的OMS-2催化剂比表面积都有增大趋势;随着CeO_2负载量的增加,低温和高温的还原峰都先向低温方向偏移再向高温方向偏移;随着CeO_2负载量的增加,高温处强酸性峰先向低温方向偏移再向高温方向偏移。反应活性结果表明,CeO_2/OMS-2催化剂活性比载体OMS-2的活性好,随着铈负载量的增加,催化活性先升高再降低,其中,CeO_2负载质量分数为1.0%(1.0Ce/OMS-2)时催化剂效果最好,这可能是由于铈的加入活化了OMS-2中的氧,影响了整体催化剂的酸性,进而影响反应性能。     

Influence of the catalyst surface area on the activity and stability of Au/CeO2 catalysts for the low-temperature water gas shift reaction

The influence of the support surface area on the activity and stability/deactivation of Au/CeO₂ catalysts (2.7 wt% Au) in the water gas shift reaction in dilute water gas were investigated by kinetic measurements and in situ Diffuse Reflectance IR spectroscopy. For ceria support surface areas between 24 and 284 m² g⁻¹, the gold particle size is independent on the catalyst surface area (about 2.1 nm) up to 188 m² g⁻¹, and we found increased amounts of (i) Auⁿ⁺, (ii) Ce³⁺, (iii) OH groups, and (iv) carbon containing adsorbed side products such as formates and carbonates for increasing surface area supports. Consequences of these results on the mechanistic understanding of the reaction are discussed.     

Au/LaCo_(1-x)Ce_xO_3催化剂的制备及其对CO催化氧化性能的研究

为进一步提高催化剂活性,用Ce对LaCoO3载体进行改性,并采用溶胶-凝胶法制备系列LaCo1-xCexO3(x=0~0.5)载体。其中,x=0.1和0.2时,载体为钙钛矿结构。采用沉积-沉淀法制备Au/LaCo1-xCexO3(x=0.1、0.2)催化剂,通过XRD、BET和H2-TPR等方法对催化剂进行催化活性评价及稳定性表征测试。结果表明,Au/LaCo0.9Ce0.1O3和Au/LaCo0.8Ce0.2O3催化剂能够在90℃将CO完全转化,在此温度进行的连续20h和30h的寿命实验中,CO转化率保持100%,催化活性和稳定性均优于Au/LaCoO3催化剂。表明掺杂Ce改性载体,能够提高催化剂活性和稳定性。