CuO CeO2 Catalysts Research Articles

Effect of doping elements on oxygen vacancies and lattice oxygen in CeO2[sbnd]CuO catalysts

Both oxygen vacancies and lattice oxygen play important roles in CeO2CuO catalysts because their concentration and distribution dominate redox properties of two oxides in hydrogen-rich streams. Herein MOX/CeO2CuO (M=Fe, Co and Ni) catalysts are prepared in order to modulate oxygen vacancies and lattice oxygen. It is found that MOX introduction increases the BET surface area of CeO2CuO catalysts and promotes the formation of Ce3+ and oxygen vacancies. The NiO/CeO2CuO exhibits the highest catalytic activity and widest temperature window of complete CO conversion, which is attributed to high concentration oxygen vacancies and lattice oxygen. The deactivation of NiO/CeO2CuO could originate from the decrease of lattice oxygen and the weakening of the interaction between CeO2 and CuO.

In Situ EPR Study of the Redox Properties of CuO–CeO2 Catalysts for Preferential CO Oxidation (PROX)

Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO–CeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce4+/Ce3+ during oxidation of Cu0/Cu+ species to Cu2+ and (ii) a direct mechanism that bypasses the redox pair Ce4+/Ce3+. In addition, EPR experiments with isotopically enriched 17O2 ...

Complete Oxidation of Benzene Over CuO-CeO2 Catalysts Prepared Using Different Process.

Catalytic combustion of benzene over CuO-CeO2 mixed oxides has been investigated. The CuO-CeO2 mixed oxides were prepared using various methods and characterized by XRD, BET and TPR. For the CuO-CeO2 catalysts with a Cu/(Cu + Ce) molar ratio of more than 0.5, highly dispersed copper oxide species were shown at 2θ = 35.5 degrees and 38.8 degrees. The CuO-CeO2 catalyst prepared using co-precipitation method showed the highest activity. In addition, the highest activity is shown on Cu0.35 (the index denotes the molar ratio Cu/(Cu + Ce)) sample and then it decreases on Cu0.5 and Cu0.6 samples.

Boosting soot combustion efficiencies over CuO–CeO2 catalysts with a 3DOM structure

A CuO–CeO2 catalyst with a well-defined 3DOM structure exhibited superior catalytic activity for soot combustion compared to its 3DOM CeO2 counterpart.

Copper-cerium oxides supported on carbon nanomaterial for preferential oxidation of carbon monoxide

AbstractThe CuxO-CeO2/Fe@CNSs, CuxO-CeO2/MWCNTs-Co and CuxO-CeO2/MWCNTs-Ni catalysts were prepared by the impregnation method and characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, H2-temperature programmed reduction and N2 adsorption-desorption techniques. It was found that the Fe nanoparticles were encapsulated into the multi-layered carbon nanospheres (CNSs). However, the multi-wall carbon nanotubes (MWCNTS) were generated on the Co/Al2O3 and Ni/Al2O3 precursor. The addition of carbon nanomaterial as supports could improve structural properties and low-temperature activity of the CuO-CeO2 catalyst, and save the used amount of metal catalysts in the temperature range with high selectivity for CO oxidation. The copper-cerium oxides supported on carbon nanomaterial had good resistence to H2O and CO2.

Important properties associated with catalytic performance over three-dimensionally ordered macroporous CeO2–CuO catalysts

Three-dimensionally ordered macroporous CeO2–CuO catalysts were prepared by the template and sol–gel method. The catalysts were characterized via SEM, TEM, XRD, H2-TPR, XPS, CO2-TPD and N2 adsorption–desorption techniques. It is found that the CeO2–CuO catalysts present the well-defined interconnected macroporous structure in three dimensions, and the skeleton of macroporous structure is composed of the CuO and CeO2 particles. Catalytic performance for preferential CO oxidation is determined by various properties including composition, structural and textural properties as well as reduction and desorption behavior. The 3DOM CeO2–CuO structure improves the interaction between CuO and CeO2, the structural and textural properties, the reduction of oxides and desorption of the adsorbed molecules on the active sites.

Catalytic performance of manganese doped CuO–CeO2 catalysts for selective oxidation of CO in hydrogen-rich gas

Manganese doped CuO–CeO2 catalysts (CuO–MnOx–CeO2) with Mn/Cu molar ratio of 1:5 and variable calcination temperatures were prepared by a hydrothermal method and used for selective oxidation of CO in hydrogen-rich gas, characterized by various techniques. An appropriate calcination temperature shows an essential stimulative effect on CuO–MnOx–CeO2 catalysts for the CO-PROX. The catalyst calcined at 500°C displays the highest low-temperature catalytic activity (T50%=74°C) and the broadest operating temperature “window” (CO conversion >99.0%, 110–140°C). XRD and UV-Raman results reveal that an appropriate calcination temperature can promote the formation of Cu–Mn–Ce–O ternary oxide solid solution, adjust the degree of crystallinity of CeO2 and enhance the formation of oxygen vacancies. H2-TPR and XPS demonstrate that calcining at 500°C improves the active reducing species in both bulk and surface of the catalyst. In situ DRIFTS suggests that Cu+ species which possess strong interaction with ceria also can be facilitated with a suitable calcination temperature.

Characterization and performance in preferential oxidation of CO of CuO–CeO2 catalysts synthesized using polymethyl metacrylate (PMMA) as template

CuO–CeO2 catalysts have been synthesized using polymethyl metacrylate (PMMA) microspheres as template and copper and cerium acetates as precursors, leading to a homogeneous porous framework of nanocrystalline materials with uniform pore size distribution and well dispersed copper species between 1 and 9 wt.%. The catalysts have been characterized by X-ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FE-SEM), High-resolution Transmission Electron Microscopy (HRTEM), Raman spectroscopy, adsorption-desorption of N2 at −196 °C, thermoprogrammed reduction with H2 (H2-TPR) and X-ray photoelectron spectroscopy (XPS). CuO–CeO2 catalysts have been evaluated in the CO-PROX reaction, reaching high conversion values (>95%) at 115 °C for all samples. Moreover, these catalysts have been tested under high content of CO2 and H2O in the feed, simulating a CO-PROX unit, obtaining conversion efficiency values higher than 90% at 115 °C, which indicates a high tolerance of the catalysts in both CO2 and H2O gases.

Wiener Model and Extremum Seeking Control for a CO Preferential Oxidation Reactor with the CuO-CeO2 catalyst

Hydrogen rich gas produced by hydrocarbon reforming contains CO up to several thousand ppm which acts as a poison for the proton exchange membrane fuel cells. Preferential oxidation (PROX) is one of the promising methods that reduce its concentration below 10ppm. The PROX reactor with the CuO-CeO2 catalyst shows optimal reactor temperature and a control system that maintains the PROX reactor temperature at its optimal one under changing environments and catalyst deactivations. Experimental studies to obtain a dynamic model for the CO PROX reactor have been done and the Wiener-type nonlinear model is obtained. Then the extremum seeking control that can track the optimal temperature for the Wiener-type nonlinear system is applied and its performance is verified experimentally. The extremum seeking control is a non-parametric real-time optimization method and can be applied effectively to other CO PROX reactors with different operating conditions.

Catalytic effect of two-phase intergrowth and coexistence CuO–CeO2

A model of the catalytic reaction mechanisms for CO + O2 and NO + CO for a two phase intergrowth and coexistence CuO–CeO2 catalyst.

Structural features and catalytic performance in CO preferential oxidation of CuO–CeO2 supported on multi-walled carbon nanotubes

MWCNT supported CuO–CeO2 catalysts show enhanced performance in CO-PROX due to unusual structure features induced by interactions between metal oxides and MWCNT.

Structure, bonding, and catalytic activity of monodisperse, transition-metal-substituted CeO2 nanoparticles.

We present a simple and generalizable synthetic route toward phase-pure, monodisperse transition-metal-substituted ceria nanoparticles (M0.1Ce0.9O2-x, M = Mn, Fe, Co, Ni, Cu). The solution-based pyrolysis of a series of heterobimetallic Schiff base complexes ensures a rigorous control of the size, morphology and composition of 3 nm M0.1Ce0.9O2-x crystallites for CO oxidation catalysis and other applications. X-ray absorption spectroscopy confirms the dispersion of aliovalent (M(3+) and M(2+)) transition metal ions into the ceria matrix without the formation of any bulk transition metal oxide phases, while steady-state CO oxidation catalysis reveals an order of magnitude increase in catalytic activity with copper substitution. Density functional calculations of model slabs of these compounds confirm the stabilization of M(3+) and M(2+) in the lattice of CeO2. These results highlight the role of the host CeO2 lattice in stabilizing high oxidation states of aliovalent transition metal dopants that ordinarily would be intractable, such as Cu(3+), as well as demonstrating a rational approach to catalyst design. The current work demonstrates, for the first time, a generalizable approach for the preparation of transition-metal-substituted CeO2 for a broad range of transition metals with unparalleled synthetic control and illustrates that Cu(3+) is implicated in the mechanism for CO oxidation on CuO-CeO2 catalysts.

Modulating the operation temperature window of CO preferential oxidation in H2-rich gases on three dimensionally ordered macroporous CeO2–CuO catalysts by tuning their composition and incooperating Fe2O3 and Co3O4

Three dimensionally ordered (3DOM) copper-ceria based catalysts (CeO2–CuO, CeO2–CuO–Fe2O3, and CeO2–CuO–Co3O4) with diverse Ce/Cu atomic ratios and different doping amounts of Fe2O3 and Co3O4 are synthesized via a precursor thermal decomposition-assisted colloidal crystal templating method. The as-obtained CeO2–CuO, CeO2–CuO–Fe2O3 and CeO2–CuO–Co3O4 catalysts possess three dimensionally ordered macroporous structures. And their particle sizes, chemical compositions, and surface elemental states are investigated according to the variations of Ce/Cu atomic ratios and doping amounts of Fe2O3 and Co3O4. The catalytic performance of CO preferential oxidation in H2-rich gases over 3DOM CeO2–CuO, CeO2–CuO–Fe2O3, and CeO2–CuO–Co3O4 catalysts are mainly relevant to their particle sizes, chemical compositions, surface states, and synergetic interactions between different components. By modulating the Ce/Cu atomic ratios and the doping amounts of Fe2O3 and Co3O4, the superior catalytic performance of 3DOM CeO2–CuO catalysts with high CO conversion, CO2 selectivity and longer-term stability in a wide operation temperature window is achieved. This synthetic strategy provides a new methodology for the fabrication of 3DOM structured highly efficient catalysts, which would be very useful in the actual working conditions for proton exchange membrane fuel cells and water-gas shift reactions.

Catalytic combustion of benzene over CuO-CeO2 mixed oxides.

Catalytic combustion of benzene over CuO-CeO2 mixed oxides has been investigated. The CuO-CeO2 mixed oxides were prepared by the combustion method using malic acid as an organic fuel and characterized by XRD, XPS and TPR. For the CuO-CeO2 catalyst with a Cu/(Cu + Ce) molar ratio of more than 0.4, highly dispersed copper oxide species were shown at 2θ = 35.5 degrees and 38.8 degrees. The CuO-CeO2 catalyst prepared using 2.0 M malic acid showed the highest activity, with conversion reaching nearly 100% at 350 degrees C. In addition, the highest activity is shown on Cu0.40 (the index denotes the molar ratio Cu/(Cu + Ce)) sample and then it decreases on Cu0.5 and Cu0.7 samples.

Mechanochemical synthesis of CuO-CeO2 catalysts for the preferential oxidation of CO in the presence of H2

Mechanochemical activation was used in the synthesis of CuO-CeO2 catalysts for the preferential oxidation of CO in the presence of excess H2. Catalysts similar in properties to supported CuO/CeO2 systems were prepared from mixtures of copper oxide (5 or 10 wt % CuO) and cerium dioxide with the use of mechanochemical activation. It was found that the time of mechanochemical activation influences the catalytic properties: a maximum conversion of CO into CO2 (97%) at 140°C was achieved with a sample of 10 wt % CuO-90 wt % CeO2 after mechanochemical activation in a ball mill for 90 min. Changes in the phase compositions of the catalysts depending on mechanochemical activation time and reaction mixture composition were studied by X-ray diffraction. The interaction of the oxides of copper and cerium in the process of mechanochemical activation with the formation of new Cu-O-Ce surface structures, which, supposedly constitute active sites for CO oxidation, was found using differential scanning calorimetry and differential thermogravimetric analysis.

Modified precipitation processes and optimized copper content of CuO–CeO2 catalysts for water–gas shift reaction

The precipitation processes of CuO–CeO2 catalysts preparation were modified, and their optimized copper content were also studied in detail. As indicated by the experimental results, the WGS catalytic activities of the CuO–CeO2 catalysts can be ranked as: stepwise precipitation (SP) > deposition–precipitation (DP) > co-precipitation (CP), suggesting that stepwise precipitation (i.e., a modified DP) is a convenient and effective method to prepare CuO–CeO2 WGS catalysts. The optimized copper contents of CuO–CeO2–SP and CuO–CeO2–CP catalysts are 20 wt.% and 25 wt.%, respectively. Their catalytic activities can be strongly correlated with the results from XRD, XPS, Raman, N2-physisorption, N2O chemisorption and H2-TPR. For DP and SP, a certain amount of copper has substitutively incorporated into ceria lattice with multiple Ce3+ and oxygen vacancies, which is considered as one relatively strong interaction between copper and ceria support. The substitutional incorporation of copper creates larger lattice distortion, lattice defect (embodying as larger lattice cell contraction of CeO2, microstrain and Raman shift) and stronger reducibility (i.e., lower reduction temperature of H2-TPR), as a consequence, higher surface energy and catalytic activity for WGS reaction. While for CP, copper nitrate and cerium nitrate were simultaneously precipitated, resulting in the burial of many copper species in ceria supports, i.e., occupancy incorporation of isolated copper into ceria lattice vacant site, which is considered as another weak interaction between copper and ceria support. Accordingly, combined with N2O chemisorption results, it is demonstrated that surface copper species of CuO–CeO2–CP are fewer and less active than those of CuO–CeO2–DP and CuO–CeO2–SP. Lastly, there is a direct relationship between the pore volume along with most probable pore size of the as-synthesized catalysts and their corresponding catalytic activities for WGS reaction.

Morphological effects of CeO2 nanostructures for catalytic soot combustion of CuO/CeO2

Rod and ellipsoid shaped CeO2 nanostructures were prepared using self-assemblies of amine surfactants as templates through the decomposition of cerium carbonate hydroxide. The morphologies of the products were greatly influenced by amine surfactants with different alkyl chain lengths. Different shaped CeO2 nanostructures supported CuO (CuO/CeO2) catalysts were prepared by a conventional impregnation method. CuO/CeO2 nanorods showed high reducibility at lower temperature, larger amount of oxygen species from H2-TPR and possessed larger external surface area and mesopore volume from N2 adsorption as compared with CuO/CeO2 with uncontrolled morphology. These catalysts showed effective soot combustion at lower temperature under tight contact condition using carbon black (CB) as a model of soot particle, as compared with CuO/CeO2 with uncontrolled morphology. Larger external surface area and mesopore volume of CuO/CeO2 catalysts contributed to the improved physical contact condition between CeO2 catalysts and aggregated CBs. Surface reducibility and the improved physical contact condition affected greatly their catalytic activity for soot combustion.

침전법으로 제조된 CuO-CeO2혼합산화물에서 벤젠의 촉매연소반응

Catalytic combustion of benzene over CuO-CeO2 mixed oxides prepared by co-precipitation method were investigated. The CuO-CeO2 mixed oxides were also prepared using different precipitant and CuO precursor. They were characterized by XRD, BET, XPS and H2-TPR. In the CuO-CeO2 catalysts, characteristic copper oxide peaks were shown at 2θ = 35.5° and 38.5° regardless of the precipitant. The Cu0.35 catalyst prepared using NH4OH as a precipitant revealed the highest activity on the combustion of benzene. In addition, the pretreatment with hydrogen enhanced the catalytic activity and the catalyst reduced at 400 ℃ showed the highest activity on the combustion of benzene.

Preferential CO oxidation (CO-PROX) catalyzed by CuO supported on nanocrystalline CeO2 prepared by a freeze-drying method

Nanocrystalline CeO2 with a regular size of 9.5nm was prepared by a freeze-drying method, and subsequent impregnated with a Cu(II) acetate solution, varying the loading of Cu (3, 6, 12wt.%). The resulting CuO/CeO2 materials were characterized by N2 physisorption at −196°C, HRTEM, H2-TPR, X-ray diffraction, Raman spectroscopy and XPS and tested as catalysts in the preferential CO oxidation in a H2-rich stream (CO-PROX) in the temperature range 40–190°C. In spite of their low specific surface areas the catalysts exhibited a good catalytic performance, resulting active and selective in the CO-PROX reaction at low temperatures. The inhibiting effect of the simultaneous presence of CO2 (15vol.%) and H2O (10vol.%) in the reaction mixture on the performance of CuO-CeO2 catalysts was also investigated. The addition of CO2 and water in the gas stream depressed CO oxidation up to 160°C, its effect being negligible at higher temperatures. Nevertheless, despite these expected deactivation phenomena, a CO conversion value higher than 90% and a CO2 selectivity of about 90% was achieved for all the samples at 160°C. The excellent performance, especially shown by the catalyst with 6wt%. of copper, has been related to the wide dispersion of the copper active sites associated with the high amount of Ce4+ species before reaction.

Low CO content hydrogen production from oxidative steam reforming of ethanol over CuO-CeO2 catalysts at low-temperature

CuO-CeO2 catalysts were prepared by a urea precipitation method for the oxidative steam reforming of ethanol at low-temperature. The catalytic performance was evaluated and the catalysts were characterized by inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, temperature-programmed reduction, field emission scanning electron microscopy and thermo-gravimetric analysis. Over CuO-CeO2 catalysts, H2 with low CO content was produced in the whole tested temperature range of 250–450 °C. The non-noble metal catalyst 20CuCe showed higher H2 production rate than 1%/oRh/CeO2 catalyst at 300–400 °C and the advantage was more obvious after 20 h testing at 400 °C. These results further confirmed that CuO-CeO2 catalysts may be suitable candidates for low temperature hydrogen production from ethanol.