CuO–CeO2 catalysts synthesized by various methods: Comparative study of redox properties

Three different preparation methods have been used to synthesize CuO–CeO₂ catalysts with 7 wt.% copper loading: coprecipitation (CP), sol–gel (SG) and urea–nitrate combustion (UC). All the samples have been characterized by a series of techniques such as XRD, Raman, TPR, TPD and OSC, in order to understand their different performance in CO oxidation, as a function of redox properties and catalyst structure. Among the catalysts, clearly CuCe-CP shows the lower activity because it presents the structure of a solid solution. CuCe-SG and CuCe-UC catalysts show much better performance in CO oxidation, in accordance to their higher redox capacity.     

Comparative study of CuO–CeO2 catalysts prepared by wet impregnation and deposition–precipitation

Two different preparation methods are used to synthesize wt. 7% CuO–CeO₂ catalysts: a conventional wet impregnation method, and a deposition–precipitation (DP) method using Na₂CO₃ as precipitating agent. Both samples are characterized by a series of techniques. CuO–CeO₂ (Cu–Ce) prepared by DP shows a lower capacity to release the lattice oxygen to form CO₂. From CO-TPR results, it is demonstrated that this catalyst is not able to reduce copper clusters at low temperatures. Also, CO-TPD shows no CO₂ formation. The activity results confirm the worse performance of Cu–Ce prepared by DP especially when oxygen is not in excess (PROX reaction with stoichometric oxygen). A copper particle size which is too small could create a stronger metal-support interaction, with lower Cu–Ce interface to react.     

SO3 decomposition over CuO–CeO2 based catalysts in the sulfur–iodine cycle for hydrogen production

The effect of several catalyst supports with large specific surface area (such as SiC, Al₂O₃, SiC–Al₂O₃–ball, and SiC–Al₂O₃) on catalytic activity was evaluated in this study. CuO–CeO₂ supported on SiC–Al₂O₃ exhibited high stability and activity, which was considerably close to the thermodynamic equilibrium curve at 625 °C during the stability test for 50 h. The SO₃ decomposition temperature decreased from 750 °C to 625 °C. SiC–Al₂O₃contained numerous micropores and mesopores and had a large specific area, indicating strong adsorption, as determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and nitrogen adsorption measurement. X-ray photoelectron spectroscopy (XPS) revealed that the surface of SiC–Al₂O₃consisted of Al₂O₃, SiC, and SiO₂ and that the cerium oxide surface had the largest number of defects. Temperature-programmed reduction (H₂-TPR) results indicated that the cerium–copper oxides on the surface of powdered SiC–Al₂O₃ had the strongest redox potential and that CuO had the lowest reduction temperature.     

Cu/MnOx–CeO2 and Ni/MnOx–CeO2 catalysts for the water–gas shift reaction: Metal–support interaction

Monometallic copper and nickel catalysts supported on cerium-manganese mixed oxides are prepared, characterized and evaluated for the Water–Gas Shift (WGS) reaction. Active metal loading of 2.5 wt% and 7.5 wt% are used to impregnate MnO_x–CeO₂ supports with 30% and 50% Mn:Ce molar ratio. The structure of the samples strongly depends on both the active metal employed and the manganese content in the mixed support. For both Cu and Ni samples, the best catalytic behavior is found in samples supported on the MnO_x–CeO₂ oxides with 30% Mn:Ce molar ratio, as a result of the presence of Cu_xMn_yO₄ spinel-type phases in the case of copper catalysts and the presence of a NiMnO₃ mixed oxide with defect ilmenite structure in the case of nickel catalysts.     

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.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Comparison of structure and catalytic performance of Pt–Co and Pt–Cu bimetallic catalysts supported on Al2O3 and CeO2 synthesized by electron beam irradiation method for preferential CO oxidation

In order to investigate the effect of transition metal addition to platinum with different support materials on preferential CO oxidation, structure and chemical properties of supported bimetallic catalysts prepared by electron beam irradiation method were correlated to the catalytic performance. On Al₂O₃, decoration of Pt by small amount of Co (Co/Pt ∼ 0.03) drastically increased CO and O₂ conversions while addition of equimolar Cu to Pt increased them only above 100 °C, where the rate-controlling factor was suggested to change from oxygen transport to CO activation. On CeO₂, either addition of Co or Cu to Pt had minor or negative effect on high O₂ conversion inherent to high oxygen transport at Pt–CeO₂ interface. On Pt–Cu/CeO₂, however, metal-CuO_x interface dominates the reaction characteristics to give improved selectivity, which is suitable for deep CO removal in excess O₂/CO condition. The order of selectivity above 100 °C, Pt–CoO_x > Pt(alloy)–CuO_x > Pt–CeO₂ interfaces, was derived from structural analysis and catalytic tests.     

Meso–macroporous monolithic CuO–CeO2/γ/α-Al2O3 catalysts for CO preferential oxidation in hydrogen-rich gas: Effect of loading methods

Meso–macroporous alumina supported CuO–CeO₂ catalysts were prepared by citrate, urea combustion and impregnation methods. The effect of loading methods on the microstructure of the catalysts, the interaction between copper and ceria and the catalytic performance for preferential oxidation of CO in hydrogen-rich gases was investigated. The prepared monolithic catalysts were characterized by using techniques of N₂ adsorption and desorption, SEM, XRD, HRTEM and TPR. The results showed that the loading methods markedly influenced the catalyst structure and the catalytic performance. The citrate and urea combustion methods favored the formation of the interaction between copper and ceria. Compared with the urea combustion method, the citrate method led to smaller ceria particles on the alumina support. The meso–macroporous monolithic catalysts prepared by the citrate method maintained the structural characteristics of the highly active CuO–CeO₂ catalysts, and showed good catalytic performance in CO preferential oxidation in the simulated reformate gases containing water and CO₂.     

CeO2–ZrO2-promoted CuO/ZnO catalyst for methanol steam reforming

The Cu-based catalysts with different supports (CeO₂, ZrO₂ and CeO₂–ZrO₂) for methanol steam reforming (MSR) were prepared by a co-precipitation procedure, and the effect of different supports was investigated. The catalysts were characterized by means of N₂ adsorption–desorption, X-ray diffraction, temperature-programmed reduction, oxygen storage capacity and N₂O titration. The results showed that the Cu dispersion, reducibility of catalysts and oxygen storage capacity evidently influenced the catalytic activity and CO selectivity. The introduction of ZrO₂ into the catalyst improved the Cu dispersion and catalyst reducibility, while the addition of CeO₂ mainly increased oxygen storage capacity. It was noticed that the CeO₂–ZrO₂-containing catalyst showed the best performance with lower CO concentration, which was due to the high Cu dispersion and well oxygen storage capacity. Further investigation illuminated that the formation of CO on CuO/ZnO/CeO₂–ZrO₂ catalyst mainly due to the reverse water gas shift. In addition, the CuO/ZnO/CeO₂–ZrO₂ catalyst also had excellent reforming performance with no deactivation during 360 h run time and was used successfully in a mini reformer. The maximum hydrogen production rate in the mini reformer reached to 162.8 dm³/h, which can produce 160–270 W electric energy power by different kinds of fuel cells.     

Effect of zirconium addition on the structure and properties of CuO/CeO2 catalysts for high-temperature water–gas shift in an IGCC system

A series of Ce_xZr_1−xO₂-based Cu catalysts was synthesized by the co-precipitation method. The influences of copper content, zirconium addition, and ratio of ceria to zirconia on the catalytic activity were investigated. BET, N₂O decomposition, XRD, TEM, SEM, EDS, Raman spectroscopy, H₂-TPR, TG/DTA, and XPS were used to characterize the catalysts. The catalytic activity was tested in terms of CO conversion and H₂ selectivity in H₂-rich coal-derived synthesis gas, simulating the actual gas composition of an integrated gasification combined cycle (IGCC) system. The long-term catalyst stability was also examined at 450 °C for 196 h. The addition of zirconium was found to be very important in enhancing catalytic performance. The surface area, copper dispersion, oxygen storage and mobility capacity, reducibility, as well as resistance to sintering all improved after zirconium addition.     

Experimental study of Ni/CeO2 catalytic properties and performance for hydrogen production in sulfur–iodine cycle

The Ni/CeO₂ catalysts with different calcination temperatures have been tested for hydrogen production in sulfur–iodine (SI or IS) cycle. TG-FTIR, BET, XRD, HRTEM and TPR were performed for catalyst characterization. It was found that the Ni²⁺ ions could be inserted into the ceria lattice. This brought about the strong interaction between Ni and CeO₂ and the generation of oxygen vacancies. Perfect crystallites were formed in the catalysts. It was evident that there was a change in particle size and morphology as the calcination temperature increased from 300 to 900 °C. The Ni/CeO₂ catalysts with different calcination temperatures showed better catalytic activity by comparison with blank yield, especially Ni/Ce700. A hypothetic mechanism of HI catalytic decomposition on Ni/CeO₂ has been constructed. The two important reactive sites were assumed for HI catalytic decomposition.     

Plasma-reduced Ni/γ–Al2O3 and CeO2–Ni/γ–Al2O3 catalysts for improving dry reforming of propane

Pristine Ni/γ–Al₂O₃ and CeO₂–Ni/γ–Al₂O₃ catalysts were prepared by co-impregnation technique for dry reforming of propane. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to examine the structure and morphology of the catalysts before and after the reforming reactions. The excellent interaction between catalyst active phases was observed in both CeO₂–Ni/γ–Al₂O₃ and Ni/γ–Al₂O₃ stabilized with polyethelene glycol (Ni/γ–Al₂O₃–PEG). Towards C₃H₈ and CO₂ conversion, the CeO₂–Ni/γ–Al₂O₃ and Ni/γ–Al₂O₃–PEG showed improved catalytic activity when compared to the pristine Ni/γ–Al₂O₃ catalyst. Interestingly, high H₂ concentration was achieved with the CeO₂–Ni/γ–Al₂O₃ and high CO concentration with the Ni/γ–Al₂O₃–PEG, which is due to the nanoconfinement of nickel particles within the support and favorable metal-support interaction as a result of plasma reduction. The CeO₂–Ni/γ–Al₂O₃ catalyst exhibited better stability for anti-sintering and coke resistance, thus exhibiting high reactivity and durability in the dry reforming.     

Hydrogen production from catalytic steam reforming of bio-oil aqueous fraction over Ni/CeO2–ZrO2 catalysts

A series of composite catalysts Ni/CeO₂–ZrO₂ were prepared via impregnation method with Ni as the active metal. A laboratory-scale fixed-bed reactor was employed to investigate the catalyst performance during hydrogen production by steam reforming bio-oil aqueous fraction. Effects of water-to-bio-oil ratio (W/B), reaction temperature, and the loaded weight of Ni and Ce on the hydrogen production performance of Ni/CeO₂–ZrO₂ catalysts were examined. The obtained results were compared with commercial nickel-based catalysts (Z417). The best performance of Ni/CeO₂–ZrO₂ catalyst was observed when the Ni and Ce loaded weight were 12% and 7.5% respectively. At W/B = 4.9, T = 800 °C, H₂ yield reaches the highest of 69.7% and H₂ content of 61.8% were obtained. Under the same condition, H₂ yield and H₂ content were higher than commercial nickel-based catalysts (Z417).     

Production of syngas via autothermal reforming of methane in a fluidized-bed reactor over the combined CeO2–ZrO2/SiO2 supported Ni catalysts

Production of syngas via autothermal reforming of methane (MATR) in a fluidized bed reactor was investigated over a series of combined CeO₂–ZrO₂/SiO₂ supported Ni catalysts. These combined CeO₂–ZrO₂/SiO₂ supports and supported Ni catalysts were characterized by nitrogen adsorption, XRD, NH₃-TPD, CO₂-TPD and H₂-TPR. It was found that the combined supports integrated the advantages of SiO₂ and CeO₂, ZrO₂. That is, they have bigger surface area (about 300 m²/g) than pure CeO₂ and ZrO₂, stronger acidity and alkalescence than that of pure SiO₂, and enhanced the mobility of H adatoms. Ni species dispersed highly on these combined CeO₂–ZrO₂/SiO₂ supports, and became more reducible. Ni catalysts on the combined supports possess higher CO₂ adsorption ability, higher methane activation ability and exhibited higher activity for MATR. H₂/CO ratio in product gas could be controlled successfully in the range of 0.99–2.21 by manipulating the relative concentrations of CO₂ and O₂ in feed.     

Steam reforming of iso-octane toward hydrogen production over mono- and bi-metallic Cu–Co/CeO2 catalysts: Structure-activity correlations

Τhe feasibility of tailoring the iso-octane steam reforming activity of Cu/CeO₂ catalysts through the use of Co as a second active metal (Cu_20−xCo_x, where x = 0, 5, 10, 15, 20 wt%), is investigated. Characterization studies, involving N₂ adsorption–desorption at −196 °C (BET), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Reduction (H₂-TPR), were carried out to reveal the impact of the morphological, structural and surface properties of the catalysts on the reforming performance. The results showed that reforming activity was monotonically increased upon increasing cobalt loading. The Co/CeO₂ catalyst demonstrated the optimum performance with a H₂ yield of 70–80% in the 600–800 °C temperature interval. The Co/CeO₂ catalyst exhibited also excellent stability at temperatures above 700 °C, while Cu-based catalysts rapidly deactivated in long term stability tests. A close correlation between surface/redox properties and steam reforming efficiency was established. The lower reducibility of Co/CeO₂ catalysts, associated with the formation of Co³⁺ species, in Co₃O₄-like phase, can be accounted for the enhanced carbon tolerance of Co-based catalysts. Furthermore, the high concentration of surface oxygen species on Co/CeO₂ catalysts can be considered for their enhanced performance. On the other hand, the Cu-induced easier reducibility of bimetallic catalysts, in conjunction with carbon deposition and active phase sintering can be accounted for their inferior steam reforming performance. Irreversible changes in the redox properties of Cu-based catalysts, taking place under reaction conditions, could be resulted to ceria deactivation thus hindering the redox process to keep on.     

Preferential CO oxidation over CuO–CeO2 in excess hydrogen: Effectiveness factors of catalyst particles and temperature window for CO removal

In the preferential oxidation of CO in hydrogen mixtures (PROX), CO and H₂ oxidation occur in parallel on the surface in a porous catalyst. The diffusion of the reactants into the pore structure of the catalyst can affect the catalyst performance significantly, and its effect can be accounted for in terms of the effectiveness factor. Conventional methods for estimating the effectiveness factor are not directly applicable because they have been developed for a single reaction in a catalyst particle. A novel method for a simultaneous estimation of the effectiveness factors of the two reactions was developed in this study. This method is based on the PROX kinetics over a CuO–CeO₂ catalyst and is applicable to the cases where the CO oxidation can be approximated by a first-order reaction and both oxidations are zero-order reactions with respect to the O₂ partial pressure. With the method, the performance of an isothermal PROX reactor was simulated to determine the effects of the feed flow rate, feed composition, reactor temperature and catalyst size on the CO clean-up.