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.     

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.     

Enhanced Activity of CuCeO Catalysts for CO Oxidation: Influence of Cu2O and the Dispersion of Cu2O,CuO, and CeO2

CuCeO catalysts prepared by a hydrothermal method with subsequent calcination are tested for the catalytic oxidation of CO. This synthesis method leads to a homogeneous dispersion of Cu₂O, CuO, and CeO₂ in the catalysts. The composition of the catalysts is determined by the molar ratio of the metals, the hydrothermal process, and calcination temperature and influences the catalytic performance. The catalyst containing Cu₂O exhibits high catalytic activity with almost 100 % CO conversion at 105 °C and shows excellent stability with the conversion ratio not decreasing after four months of storage.     

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 oxidation of CO in the presence of hydrogen on CuO/CeO<Subscript>2</Subscript> catalysts modified with Fe and Ni oxides

The selective oxidation of CO in the presence of hydrogen on CuO/CeO₂ systems containing Fe and Ni oxides as promoters was studied. The catalysts containing 1–5 wt % CuO and 1–2.5 wt % Fe₂O₃ supported on CeO₂ and the CuO/CeO₂ systems containing 1–2.5 wt % NiO were synthesized, and their catalytic activity as a function of temperature was determined. It was found that the additives of Fe and Ni oxides increased the activity of the CuO/CeO₂ catalysts with a low concentration of CuO. In this case, the conversion of CO at 150°C approached 100%. At the same time, these additives had no effect on the activity of the CuO/CeO₂ systems at a CuO concentration of 5 wt % or higher, which exhibited an initially high activity in the above temperature region. The forms of CO adsorption and the amounts of active sites for CO adsorption and oxidation were studied using temperature-programmed desorption. It was found that the introduction of Fe and Ni additives in a certain preparation procedure facilitated the formation of an additional amount of active centers associated with CuO. Data on the temperature-programmed reduction of samples (the amount of absorbed hydrogen and the maximum temperature of hydrogen absorption) suggested the interaction of all catalyst components, and the magnitude of this interaction depended on the sample preparation procedure. With the use of Mössbauer spectroscopy, it was found that the procedure of iron oxide introduction into the CuO/CeO₂ system was responsible for the electron-ion interactions of catalyst components and the reaction mixture.     

CO oxidation with oxygen of the catalyst and gas-phase oxygen over (0.5−15)%СоО/СеО<Subscript>2</Subscript>

The CO adsorption species on Co₃O₄ and (0.5-15%)CoO/CeO₂ catalysts have been investigated by temperature-programmed desorption and IR spectroscopy. At 20°C, the largest amount of CO is adsorbed on the 5%CoO/CeO₂ sample to form, on Co_m²⁺O_n²⁺ clusters, hydrogen-containing, bidentate, and monodentate carbonate complexes, whose decomposition is accompanied by CO₂ desorption at 300 and 450°C (1.1 × 10²⁰ g^–1). The formation of the carbonates is accompanied by the formation of Co⁺ cations and Co⁰, on which carbonyls form. The latter decompose at 20, 90, and 170°C to release CO (2.7 × 10¹⁹ g^–1). Part of the carbonyls oxidizes to CO₂ upon oxygen adsorption, and the CO₂ undergoes desorption at 20°C. Adsorbed oxygen decreases the decomposition temperature of the H-containing and bidentate carbonates from 300 to 100-170°C and maintains the sample in the oxidized state, which is active in subsequent CO adsorption and oxidation. CO oxidation by oxygen of the catalyst diminishes the activity of the sample in these processes and increases the decomposition temperature of the carbonate complexes. Taking into account the properties of the adsorption complexes, we concluded that the H-containing and bidentate carbonates are involved in CO oxidation by oxygen of the catalyst at ~170°C under isothermal conditions. The rate limiting step is the decomposition of the carbonates, a process whose activation energy is 65-74 kJ/mol.     

The Effect of the Copper Oxide Content and Support Structure in (0.5−15%)СuО/ZrO<Subscript>2</Subscript> Catalysts on Their Activity in the CO Oxidation Reaction with Oxygen in an Excess of Hydrogen

The dependence of the activity of СuO/ZrO₂ catalysts in the CO oxidation reaction with oxygen in the presence of an excess of hydrogen and adsorption of СО over them on the CuO content (0.5 to 15%) and the structure of the support ZrO₂, monoclinic (М), tetragonal (Т), or mixed (М + Т) has been studied. It has been found that the activity of CuO/ZrO₂ is associated with the adsorption capacity of the samples for СО at 20°С. Thus, 5%CuO/ZrO₂(Т + М) and 5% CuO/ZrO₂(Т) samples, which exhibit the maximum activity (the СО conversion over them is 80–85% at 160°С), also possess a high chemisorption capacity towards CO (~2.2 × 10²⁰ molecules/g). At the same time, CuO/ZrO₂(М) samples with the CuO contents of 1 and 5% do not chemisorb СО and are inactive in the reaction at 160°С. The СО conversion over them does not exceed 32–36% at 250°С. On the basis of the data obtained by X-ray phase analysis, temperature-programmed reduction with Н₂, temperature-programmed СО desorption, and electron paramagnetic resonance, a conclusion has been made that at low temperatures СО oxidation proceeds over Cu_nO_m clusters that are located on ZrO₂(Т) crystallites. With the increase in the copper oxide content from 0.5 to 5%, the activity of the clusters increases, while the reaction temperature decreases. CuO_m oxo complexes and particles of the СuO phase do not exhibit catalytic activity. The reasons for the low activity of the CuO/ZrO₂(М) samples with the CuO contents of 1 and 5% in the СО oxidation and adsorption processes are discussed. The mechanism of the low-temperature СО oxidation with oxygen in an excess of hydrogen over a 5% CuO/ZrO₂(Т + М) catalyst is considered.     

制备方法对CuO/CeO_2-ZrO_2催化CO低温氧化活性的影响

采用微波加热分解法(一步法)和微波加热处理共沉淀+浸渍法(两步法)制备了CuO/CeO2-ZrO2催化剂,并对其进行了X射线衍射、低温氮气吸附/脱附和程序升温还原等表征,采用色谱流动法考察了催化剂的催化CO低温氧化性能.结果表明,一步法比两步法更有利于使催化剂表面CuO高度分散,CuO与CeO2-ZrO2间的相互作用更强,CuO更容易被还原,从而具有更高的催化CO氧化活性.与CeO2-ZrO2有相互作用的高分散和小颗粒CuO有利于催化剂活性的提高,与CeO2-ZrO2无相互作用的大颗粒CuO对催化剂的活性有抑制作用.     

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.     

Efficient electroreduction of CO2 to C2+ products on CeO2 modified CuO

Electrocatalytic reduction of CO₂ into multicarbon (C₂₊) products powered by renewable electricity offers one promising method for CO₂ utilization and promotes the storage of renewable energy under an ambient environment. However, there is still a dilemma in the manufacture of valuable C₂₊ products between balancing selectivity and activity. In this work, cerium oxides were combined with CuO (CeO₂/CuO) and showed an outstanding catalytic performance for C₂₊ products. The faradaic efficiency of the C₂₊ products could reach 75.2% with a current density of 1.21 A cm⁻². In situ experiments and density functional theory (DFT) calculations demonstrated that the interface between CeO₂ and Cu and the subsurface Cu₂O coexisted in CeO₂/CuO during CO₂RR and two competing pathways for C–C coupling were promoted separately, of which hydrogenation of *CO to *CHO is energetically favoured. In addition, the introduction of CeO₂ also enhanced water activation, which could accelerate the formation rate of *CHO. Thus, the selectivity and activity for C₂₊ products over CeO₂/CuO can be improved simultaneously.

CO₂ can be efficiently converted into C₂₊ products on CeO₂ modified CuO catalysts and the faradaic efficiency could reach 75.2% with a current density of 1.21 A cm⁻².     

Copper doped ceria porous nanostructures towards a highly efficient bifunctional catalyst for carbon monoxide and nitric oxide elimination

Copper doped ceria porous nanostructures with a tunable BET surface area were prepared using an efficient and general metal–organic-framework-driven, self-template route. The XRD, SEM and TEM results indicate that Cu²⁺ was successfully substituted into the CeO₂ lattice and well dispersed in the CeO₂:Cu²⁺ nanocrystals. The CeO₂:Cu²⁺ nanocrystals exhibit a superior bifunctional catalytic performance for CO oxidation and selective catalytic reduction of NO. Interestingly, CO oxidation reactivity over the CeO₂:Cu²⁺ nanocrystals was found to be dependent on the Cu²⁺ dopants and BET surface area. By tuning the content of Cu²⁺ and BET surface area through choosing different organic ligands, the 100% conversion temperature of CO over CeO₂:Cu²⁺ nanocrystals obtained from thermolysis of CeCu–BPDC nanocrystals can be decreased to 110 °C. The porous nanomaterials show a high CO conversion rate without any loss in activity even after five cycles. Furthermore, the activity of the catalysts for NO reduction increased with the increase of BET surface, which is in accordance with the results of CO oxidation.     

Dispersion state of CuO on CeO2

XRD and XPS are used to study the dispersion state of CuO on ceria surface. The dispersion capacity values of CuO measured by the two methods are consistent, which are of 1.20 mmol CuO/100 m² CeO₂. In addition, the results reveal that highly dispersed Cu²⁺ ions are formed at low CuO loadings and that increasing the CuO content to a value higher than its dispersion capacity produces crystalline CuO after the surface vacant sites on CeO₂ are filled. The atomic composition of the outermost layer of the CuO/CeO₂ samples has been probed by using static secondary ion mass spectroscopy (SSIMS), and the ratim of Cu/Ce are found to be 0.93 and 0.46 for the 1.22 and 0.61 mmol CuO/CeO₂ samples respectively. Temperature-programmed reduction (TPR) profile with two reduction peaks at 156 and 165°C suggests that the reduction of highly dispersed Cu²⁺ ions consists of two steps and is easier than that of CuO crystallites, in which the TPR profile has only one reduction peak at about 249°C. The above experimental results are in good agreement with the prediction of the incorporation model. Project supported by the National Natural Science Foundation of China.     

CuO/Ti0.5Zr0.5O2催化剂对NO+CO反应的催化作用

环境治理是当今社会面临的一大主要问题。目前,城市空气污染日趋严重,特别是工厂和汽车排放的大量未燃烧的烃类、CO、NOx是主要的空气污染物。其中,氮氧化物(NOx)排放状况尤其严重,它的排放会给环境和人们生活带来严重危害,因此,如何有效地消除NOx已成为目前环境保护中一个非常     

Construction of Cu-Ce interface for boosting toluene oxidation: Study of Cu-Ce interaction and intermediates identified by in situ DRIFTS

A facile hydrothermal method was applied to gain stably and highly efficient CuO-CeO₂ (denoted as Cu1Ce2) catalyst for toluene oxidation. The changes of surface and inter properties on Cu1Ce2 were investigated comparing with pure CeO₂ and pure CuO. The formation of Cu-Ce interface promotes the electron transfer between Cu and Ce through Cu²⁺ + Ce³⁺ ↔ Cu⁺ + Ce⁴⁺ and leads to high redox properties and mobility of oxygen species. Thus, the Cu1Ce2 catalyst makes up the shortcoming of CeO₂ and CuO and achieved high catalytic performance with T₅₀ = 234 °C and T₉₉ = 250 °C (the temperature at which 50% and 90% C₇H₈ conversion is obtained, respectively) for toluene oxidation. Different reaction steps and intermediates for toluene oxidation over Cu1Ce2, CeO₂ and CuO were detected by in situ DRIFTS, the fast benzyl species conversion and preferential transformation of benzoates into carbonates through C=C breaking over Cu1Ce2 should accelerate the reaction.     

NOx-Sorbing Metal Oxides, MnOx–CeO2. Oxidative NO Adsorption and NOx–H2 Reaction

(n)MnO_x–(1?n)CeO₂ binary oxides have been studied for the sorptive NO removal and subsequent reduction of NO_x sorbed to N₂ at low temperatures (≤150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO^? ₃) and/or nitrite (NO^? ₂) species on the surface depending on temperature, O₂ concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NO_x toward H₂ was examined for MnO_x–CeO₂ impregnated with Pd, which is known as a nonselective catalyst toward NO–H₂ reaction in the presence of excess oxygen. The Pd/MnO_x–CeO₂ catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H₂ at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnO_x–CeO₂ surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H₂–O₂ combustion, H₂–NO reaction was suppressed to a certain extent in the presence of O₂. Nevertheless, Pd/MnO_x–CeO₂ attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H₂, and 6% O₂ in He at as low as 150 °C, compared to ca. 30% conversion for Pd/γ–Al₂O₃ at the same temperature. The combination of NO_x-sorbing materials and H₂-activation catalysts is expected to pave the way to development of novel NO_x-sorbing catalysts for selective deNO_x at very low temperatures.     

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.     

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.     

CO oxidation by oxygen of the catalyst and by gas-phase oxygen over (0.5–15)%CoO/ZrO<Subscript>2</Subscript>

CO adsorption on (0.5–15)%CoO/ZrО₂ catalysts has been investigated by temperature-programmed desorption and IR spectroscopy. At 20°С, carbon monoxide forms carbonyl and monodentate carbonate complexes on Co _m ²⁺ O _n ^2- clusters located on the surface of crystallites of tetragonal ZrO₂. With an increasing CoO content of the clusters, the amount of these complexes increases and the temperature of carbonate decomposition, accompanied by CO₂ desorption, decreases from 400 to 304°С. On the 5%CoO/ZrО₂ sample, the carbonyls formed on the Со²⁺ and Со⁺ cations and Со⁰ atoms decompose at 20, 90, and 200–220°С, respectively, releasing CO. At 20°С, they are oxidized by oxygen to monodentate carbonates, which decompose at 180°С. Adsorbed oxygen decreases the temperature of their decomposition on oxidation sites by ~40°C, and the sample remains in an oxidized state ensuring the possibility of subsequent CO adsorption and oxidation. The rate of the oxidation of 5%CoO/ZrО₂ containing adsorbed CO by oxygen is higher than the rate of the oxidation of the same sample reduced by carbon monoxide, because the latter reaction is an activated one. In view of the properties of the complexes, it can be concluded that the carbonates decomposing at 180°С are involved in CO oxidation by oxygen from the gas phase in the presence of hydrogen, a process occurring at 50–200°С. The rate-limiting step of this process the decomposition of the carbonates, which is characterized by an activation energy of 77–94 kJ/mol.     

The relationship of morphology and catalytic performance of CeO_2 catalysts for reducing nitrobenzene to azoxybenzene under the base-free condition

CeO_2 morphology was proposed to be a crucial factor for reducing nitrobenzene to azoxybenzene under the base-free condition.Herein,the structure-activity relationship of CeO_2 catalysts was explored to improve the azoxybenzene yield.A series of CeO_2 catalysts we re synthesized with seven morphologies to obtain different Ce~(3+) proportion and various surface areas.Notably,the catalytic performance of these samples for reducing nitrobenzene to azoxybenzene enhanced with the increasing Ce~(3+) proportio n.With the highest surface Ce~(3+)proportion,the Rod-CeO_2 catalyst exhibited 100% conversion of nitrobenzene and 89.8% azoxybenzene selectivity in 7 h at 150℃ under 1 MPa CO.Moreover,the preliminary mechanistic analysis indicated that the inhabitation of azoxybenzene to by-product azobenzene resulted in the high selectivity of azoxybenzene.     

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.