Co oxidation with oxygen in the presence of hydrogen on CoO/CeO<Subscript>2</Subscript> and CuO/CoO/CeO<Subscript>2</Subscript> catalysts

The catalytic activity of the CoO/CeO₂ and CuO/CoO/CeO₂ systems in selective CO oxidation in the presence of hydrogen at 20–450°C ([CuO] = 1.0–2.5%, [CoO] = 1.0–7.0%) is reported. The maximum CO conversion (X) decreases in the following order: CuO/CoO/CeO₂ (X = 98–99%, T = 140–170°C) > CoO/CeO₂ (X = 67–84%, T = 230–240°C) > CeO₂ (X = 34%, T = 350°C). TPD, TPR, and EPR experiments have demonstrated that the high activity of CuO/CoO/CeO₂ is due to the strong interaction of the supported copper and cobalt oxides with cerium dioxide, which yields Cu-Co-Ce-O clusters on the surface. The carbonyl group in the complexes Co^δ+-CO and Cu⁺-CO is oxidized by oxygen of the Cu-Co-Ce-O clusters at 140–160°C and by oxygen of the Co-Ce-O clusters at 240°C. The decrease in the activity of the catalysts at high temperatures is due to the fact that hydrogen reduces the clusters on which CO oxidation takes place, yielding Co⁰ and Cu⁰ particles, which are inactive in CO oxidation. The hydrogenation of CO into methane at high temperatures is due to the appearance of Co⁰ particles in the catalysts.     

Preparation,characterization and catalytic applications of ZrO<Subscript>2</Subscript> supported on low cost SBA-15

This work presents some applications of ZrO₂ supported over SBA-15 silica as promoter of sulfated zirconia and as support from CuO/CeO₂ catalytic system for preferential oxidation of CO to CO₂ in hydrogen rich streams, used as feed for proton exchange membrane fuel cells (PEMFC). Different amounts of ZrO₂, from 10 to 30 wt.% were incorporated. These prepared materials were characterized by powder XRD, adsorption-desorption of N₂ at 77 K, transmission and scanning electron microscopy (TEM and SEM) and X-rays photoelectron spectroscopy (XPS). The acidity was studied by thermo-programmed desorption of ammonia (NH₃-TPD). These materials were tested, after treatment with H₂SO₄, by 2-propanol dehydration and 1-butene isomerization catalytic tests. The samples were found quite good catalyst with strong acid sites, the sample with 20 wt.% of ZrO₂ being the better performing sample. Finally this material was successfully used as support for a CuO/CeO₂ system, with 6 wt.% of Cu and 20 wt.% of Ce. The resulting catalyst was tested in the preferential oxidation of CO (CO-PROX) attaining conversions close to 100% and high selectivity to CO₂.     

Effect of the microstructure of the supported catalysts CuO/TiO<Subscript>2</Subscript> and CuO/(CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript>) on their catalytic properties in carbon monoxide oxidation

The effect of the microstructure of titanium dioxide on the structure, thermal stability, and catalytic properties of supported CuO/TiO₂ and CuO/(CeO₂-TiO₂) catalysts in CO oxidation was studied. The formation of a nanocrystalline structure was found in the CuO/TiO₂ catalysts calcined at 500°C. This nanocrystalline structure consisted of aggregated fine anatase particles about 10 nm in size and interblock boundaries between them, in which Cu²⁺ ions were stabilized. Heat treatment of this catalyst at 700°C led to a change in its microstructure with the formation of fine CuO particles 2.5–3 nm in size, which were strongly bound to the surface of TiO₂ (anatase) with a regular well-ordered crystal structure. In the CuO/(CeO₂-TiO₂) catalysts, the nanocrystalline structure of anatase was thermally more stable than in the CuO/TiO₂ catalyst, and it persisted up to 700°C. The study of the catalytic properties of the resulting catalysts showed that the CuO/(CeO₂-TiO₂) catalysts with the nanocrystalline structure of anatase were characterized by the high-est activity in CO oxidation to CO₂.     

Selective methanation of CO in the presence of CO<Subscript>2</Subscript> in hydrogen-containing mixtures on nickel catalysts

The screening of commercial nickel catalysts for methanation and a series of nickel catalysts supported on CeO₂, γ-Al₂O₃, and ZrO₂ in the reaction of selective CO methanation in the presence of CO₂ in hydrogen-containing mixtures (1.5 vol % CO, 20 vol % CO₂, 10 vol % H₂O, and the balance H₂) was performed at the flow rate WHSV = 26000 cm³ (g Cat)⁻¹ h⁻¹. It was found that commercial catalytic systems like NKM-2A and NKM-4A (NIAP-07-02) were insufficiently effective for the selective removal of CO to a level of <100 ppm. The most promising catalyst is 2 wt % Ni/CeO₂. This catalyst decreased the concentration of CO from 1.5 vol % to 100 ppm in the presence of 20 vol % CO₂ in the temperature range of 280–360°C at a selectivity of >40%, and it retained its activity even after contact with air. The minimum outlet CO concentration of 10 ppm at 80% selectivity on a 2 wt % Ni/CeO₂ catalyst was reached at a temperature of 300°C.     

Effect of nickel oxide additive on properties of catalysts used in the reaction of selective oxidation of carbon monoxide

Effect of nickel oxide additives on the oxidation selectivity of carbon monoxide in the presence of hydrogen was studied for the widely recognized system constituted by copper and cerium oxides. It was shown that a significant positive effect is observed upon introduction of the nickel-containing additive (??0.3%) due to the electron-donor effect of nickel oxide. It was demonstrated that the catalyst of composition NiO/CuO/CeO₂/Al₂O₃ shows high selectivity in the reaction of CO oxidation.     

Effect of the nanoparticle size of supported copper-containing catalysts on their activity in the selective oxidation of CO in an excess of H<Subscript>2</Subscript>

The catalytic properties of systems prepared by the supporting of CuO onto CeO₂, ZrO₂, and Zr_0.5Ce_0.5O₂ with particle sizes of 15–25 nm (nitrate pyrolysis (p)) and 5–6 nm (microemulsion method (me)) in the reaction of CO oxidation in an excess of H₂ were studied. In the latter case, the supports had an almost homogeneous surface and a small number of defects. The catalytic activity of (me) and (p) supports was low and almost the same, whereas the catalytic activity of CuO/(CeO₂, ZrO₂, and Zr_0.5Ce_0.5O₂)(me) samples was lower than that of CuO/(CeO₂ and ZrO₂)(p). The maximum CO conversion (∼100% at 125°C) was observed on 5% CuO/CeO₂ (p). The CO and CO₂ adsorption species on (p) and (me) catalysts were studied by TPD. Differences in the compositions of copper-containing centers on the surfaces of (p) and (me) systems were found using TPR. The nature of the active centers of CO oxidation and the effect of support crystallite size on the catalytic activity were considered.     

Mechanochemical Activation of Cu–CeO<Subscript>2</Subscript> Mixture as a Promising Technique for the Solid-State Synthesis of Catalysts for the Selective Oxidation of CO in the Presence of H<Subscript>2</Subscript>

A new ecologically clean method for the solid-phase synthesis of oxide copper–ceria catalysts with the use of the mechanochemical activation of a mixture of Cu powder (8 wt %) with CeO₂ was developed. It was established that metallic copper was oxidized by oxygen from CeO₂ in the course of mechanochemical activation. The intensity of a signal due to metallic Cu in the X-ray diffraction analysis spectra decreased with the duration of mechanochemical activation. The Cu¹⁺, Cu²⁺, and Ce³⁺ ions were detected on the sample surface by X-ray photoelectron spectroscopy. The application of temperature-programmed reduction (TPR) made it possible to detect two active oxygen species in the reaction of CO oxidation in the regions of 190 and 210–220°C by a TPR-H₂ method and in the regions of 150 and 180–190°C by a TPR-CO method. It is likely that the former species occurred in the catalytically active nanocomposite surface structures containing Cu–O–Ce bonds, whereas the latter occurred in the finely dispersed particles of CuO on the surface of CeO₂. The maximum conversion of CO (98%, 165°C) reached by the mechanochemical activation of the sample for 60 min was almost the same as conversion on a supported CuO/CeO₂ catalyst.     

Temperature-programmed reduction of lightly yttrium-doped Au/CeO<Subscript>2</Subscript> catalysts

The reducibility of Au catalysts on CeO₂ supports doped with 1 and 2.5 mass% Y₂O₃ by two types of preparation methods (impregnation and co-precipitation) has been studied by temperature-programmed reduction and compared with that of pure Au/CeO₂. The kinetic parameters of reduction were determined simulating each reduction process. The capacities of these catalysts to retain oxygen have been evaluated by temperature-programmed desorption. The catalytic activities in water gas shift reaction were determined measuring CO conversion between 413 and 623 K. The catalytic performances of all these catalysts were explained in terms of mobility of the oxygen ions of the CeO₂ lattice.     

Synthesis of highly ordered mesoporous CeO<Subscript>2</Subscript> and low temperature CO oxidation over Pd/mesoporous CeO<Subscript>2</Subscript>

Highly ordered mesoporous cerium dioxide (meso-CeO₂) was successfully synthesized using a facile solvent-free infiltration method from a mesoporous silica template, KIT-6. The meso-CeO₂ material, thus obtained, exhibited well-defined mesostructure and high surface area (153 m² g⁻¹). The physicochemical properties of meso-CeO₂ material and Pd-supported on meso-CeO₂ (Pd/meso-CeO₂) were characterized by electron microscopy, X-ray diffraction, N₂ adsorption–desorption, and temperature-programmed experiments. The Pd/meso-CeO₂ catalyst exhibited excellent catalytic activity for CO oxidation compared with those of other Pd/CeO₂ catalysts which were prepared using nanocrystalline CeO₂ and bulk-CeO₂ as the supports. Moreover, a hydrogen pretreatment of the Pd/meso-CeO₂ catalyst resulted in a remarkable increase of catalytic activity (T ₁₀₀ = 52 °C).     

CO oxidation over Au/CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> catalists: The effect of the support composition of the au-support interaction

The effect of the support composition on the Au-support interactions and its role in the creation of the activity of Au/CeO₂-ZrO₂ catalysts in CO oxidation has been studied. The CeO₂-ZrO₂ oxides and Au/CeO₂-ZrO₂ catalysts were synthesized, characterized by BET, XRD, HRTEM, AAS, TPR-H₂, and tested in CO oxidation. An approximate evaluation of the H₂ consumption for the surface reduction of the studied samples was estimated applying the model developed by Johnson and Mooi, which is based on the qualitative relationship between the amount of the capping oxygen and BET surface area. The sequence of the increasing percentage of O₂ atoms in the capping peak to the total Ce atoms follows the sequence of the decreasing Zr/Ce molar ratio in the sample. The activity of Au/CeO₂-ZrO₂ catalysts depends on the support composition and increases with the decrease in Zr/Ce molar ratio.     

Methanol conversion over CuO supported on CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript>: An in situ IR spectroscopic study

The main reactions yielding hydrogen are the recombination of hydrogen atoms on copper clusters and methyl formate decomposition. Methyl formate results from the interaction between the linear methoxy group and the formate complex located on CuO. The source of CO₂ appearing in the gas phase is the formate complex, and the source of CO is methyl formate. The rates of methoxy group conversion and product formation over supports (ZrO₂, CeO₂, Ce_0.8Zr_0.2O₂) and copper-containing catalysts (5%Cu/CeO₂, 5%Cu/ZrO₂, 2%Cu/Ce_0.8Zr_0.2O₂, 2%Cu/Ce_0.1Y_0.1Zr_0.8) are compared. The dominant process in methoxy group conversion over the supports and copper-containing catalysts is methanol decomposition to H₂ and CO and to H₂ and CO₂, respectively. The methoxy group conversion rate is proportional to the H₂ and CO₂ formation rate and is determined by the concentration of supported copper.     

Fabrication of Cu-Doped CeO<Subscript>2</Subscript> Catalysts with Different Dimension Pore Structures for CO Catalytic Oxidation

Transition metal oxides (TMOs) applied as catalysts whose catalytic activities are directly affected by their pores size and pores distributions. Herein, two-dimensional Cu-doped CeO₂ (2D@Cu–CeO₂) and three-dimensional Cu-doped CeO₂ (3D@Cu–CeO₂) were prepared by adopting the mesoporous silica SBA-15 and KIT-6 as templates, respectively. Nanometer Cu-doped CeO₂ (nano@Cu–CeO₂) was synthesized by the method of precipitation. All catalysts were evaluated for the catalytic oxidation of CO, and the 3D@Cu–CeO₂ catalyst exhibited the highest catalytic activity (complete conversion temperature T₁₀₀?=?50?°C), which can be ascribed to the three-dimensional porous channel structure, larger specific surface area and abundant active surface oxygen species. In addition, complete conversion of CO had remained the same after 3D@Cu–CeO₂ was observed for 12 h, indicating it has the best catalytic stability for CO.     

Effect of decomposition of catalyst precursor on Ni/CeO2 activity for CO methanation

CO methanation on Ni/CeO₂ has recently received increasing attention. However, the low-temperature activity and carbon resistance of Ni/CeO₂ still need to be improved. In this study, plasma decomposition of nickel nitrate was performed at ca. 150°C and atmospheric pressure. This was followed by hydrogen reduction at 500 °C in the absence of plasma, and a highly dispersed Ni/CeO₂ catalyst was obtained with improved CO adsorption and enhanced metal-support interaction. The plasma-decomposed catalyst showed significantly improved low-temperature activity with high methane selectivity (up to 100%) and enhanced carbon resistance for CO methanation. For example, at 250°C, the plasma-decomposed catalyst showed a CO conversion of 96.8% with high methane selectivity (almost 100%), whereas the CO conversion was only 14.7% for a thermally decomposed catalyst.     

固相浸渍法和湿浸渍法制备CuO/CeO2催化剂及其CO氧化性能的对比研究

采用固相浸渍法和常规湿浸渍法制备了一系列CuO/CeO₂催化剂,并结合X射线衍射（XRD）、氢气-程序升温还原（H₂-TPR）、激光拉曼光谱（LRS）、原位漫反射红外光谱（in situ DRIFTS）、X射线光电子能谱（XPS）等手段考察了制备方法对催化剂结构性质及其在CO氧化反应中性能的影响. XPS和H₂-TPR结果表明,固相浸渍法更有利于得到高分散的铜物种,并促进CuO物种的还原. LRS结果表明,相比于湿浸渍法,固相浸渍法能产生更多氧空位,而这些氧空位可以活化参与反应的O₂. CO氧化活性测试结果表明,当铜负载量相同时,固相浸渍法制备的催化剂相比于湿浸渍法表现出更好的催化性能. 结合多种表征结果发现,催化剂CO氧化性能与其表面氧空位和Cu⁺-CO浓度紧密相关,提出了CuO/CeO₂催化剂在CO氧化反应中可能的协同作用机制.     

CeO<Subscript>2</Subscript>-catalyzed ozonation of phenol

Three different cerium citrate-based precursors were used for synthesizing CeO₂ through thermal treatment. Three morphological types of CeO₂ were obtained. Characterization of these oxides was carried out by XRD patterns, SEM microscopy, N₂ adsorption isotherms, Raman spectroscopy, zeta potential, and UV/Vis luminescence. Ozonation of phenol catalyzed by CeO₂ was studied as a representative reaction of environmental interest. The differences on the catalytic activity showed by these three oxides could be correlated to amounts of Ce³⁺ on CeO₂ surface and, consequently, to the demand for oxygen needed to burn each precursor.     

Effect of transition metal oxide additives on the activity of an Ag/SiO2 catalyst in carbon monoxide oxidation

The catalysts of silver supported on mesoporous silica modified with Co₃O₄, CeO₂, and ZrO₂ were prepared by an impregnation method; characterized by X-ray diffraction analysis, temperature-programmed reduction, and low-temperature nitrogen adsorption; and studied in a model reaction of CO oxidation. It was found that the Ag/SiO₂ system exhibited high activity in the reaction of CO oxidation, and the addition of transition metal oxides led to reduction of the temperature of 50% CO conversion by 40°C. The modification of Ag/SiO₂ with cerium dioxide was found most effective because of the interaction of silver particles and CeO₂ on the surface of silica gel.     

Supported CuO/Ce1−x Zr x O2 catalysts for the preferential oxidation of CO in H2-rich gases

The catalytic properties of CuO supported on ceria or ceria-zirconia mixed oxides have been investigated in the preferential oxidation of CO in H₂-rich gases. CuO/CeO₂ shows very high activity towards the oxidation of CO with a light-off temperature of about 70°C. This catalyst is very selective for the oxidation of CO rather than of H₂ in the low temperature region (70–120°C), while at higher temperatures, the oxidation of hydrogen begins, causing of a maximum of CO conversion to arise with increasing temperature. Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 5, pp. 779–787. This article was submitted by the authors in English.     

Effect of Pd and rare earth oxides (La<Subscript>2</Subscript>O<Subscript>3</Subscript>, CeO<Subscript>2</Subscript>) on the properties of a Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/cordierite catalyst in reduction of O<Subscript>2</Subscript> and no by hydrogen

We have established that introducing a promoter (Pd) and modifying additives (La₂O₃, CeO₂) into the composition of a Co₃O₄/cordierite catalyst leads to an increase in its activity and selectivity during reduction of oxygen by hydrogen in the presence of nitrogen(II) oxide.     

负载Ni催化剂用于乙二醇蒸汽重整制氢催化性能研究

采用等体积浸渍法和共沉淀法制备了Ni催化剂,在固定床反应器上考察了Ni负载量、焙烧温度、反应温度等因素对乙二醇低温重整制氢反应活性和选择性的影响。应用X射线衍射、氮物理吸附、H₂程序升温还原等技术对负载型Ni催化剂进行了表征。结果表明,共沉淀法制备的Ni/CeO₂催化剂具有较小的NiO颗粒与CeO₂载体颗粒粒径,催化活性较高。添加少量氧化钴到Ni/CeO₂催化剂中可使H₂收率达72.6%,EG转化率达93.1%。在CeO₂中添加Al₂O₃能提高负载Ni催化剂的活性,乙二醇转化率达94.0%,H₂收率达67.0%;但添加SiO₂则使其活性明显变差。     

Ni/CeO2-SiO2催化剂的制备、表征及其甲烷部分氧化制合成气性能

以硝酸亚铈（Ce（NO₃）₃·6H₂O）和正硅酸四乙酯（C₈H₂₀O₄Si）为前驱体,采用溶胶-凝胶法合成了系列具有大比表面积的xCeO₂-（1-x）SiO₂（x = 0,0.25,0.50,0.75,1）复合氧化物载体,然后浸渍活性组分Ni制得用于甲烷部分氧化制合成气的Ni催化剂。运用N₂物理吸附-脱附、X射线粉末衍射、扫描电镜、紫外-可见漫反射光谱、氢程序升温还原、氨程序升温脱附和热重等手段对所得催化剂的组织结构、还原性、表面酸性和积炭行为等进行了表征;同时考察了催化剂的组成、焙烧温度和反应时间等对催化剂在甲烷部分氧化制合成气中催化性能的影响。表征结果表明,该系列Ni/CeO₂-SiO₂催化剂具有大比表面积,CeO₂晶粒较小,NiO的分散性好且易被还原,表面酸性弱,不容易积炭。当Ce/Si摩尔比为1：1,活性组分Ni的质量分数为10%,焙烧温度为700℃时,所制备的Ni/CeO₂-SiO₂催化剂表现出较好的稳定性、最高的CH₄转化率（~84%）和对产物CO及H₂的选择性（>87%）。