CuO/CeO2 supported on Zr doped SBA-15 as catalysts for preferential CO oxidation (CO-PROX)

The application of the catalytic system CuO/CeO₂ supported on Zr doped SBA-15 mesoporous silica to the preferential oxidation of CO on hydrogen streams (CO-PROX) suitable to be used to feed PEM fuel cells, has been studied. A loading of 20% (wt.) Ce and 6% (wt.) Cu was found optimal for the CO-PROX reaction. The influence of the presence of CO₂ and H₂O in the gas feed was also studied in order to simulate the real operation conditions of a PEMFC feed stream generated by alcohol steam reforming. The catalysts were characterized by XRD, adsorption-desorption of N₂ at −196 °C, TEM, -H₂-TPR and XPS. The system reducibility was found modified by the incorporation of zirconium in the support, with improvement of both the conversion and selectivity of the catalytic system, compared to the same material without Zr.     

Comparative study of CeO2/CuO and CuO/CeO2 catalysts on catalytic performance for preferential CO oxidation

The CeO₂/CuO and CuO/CeO₂ catalysts were synthesized by the hydrothermal method and characterized via XRD, SEM, H₂-TPR, HRTEM, XPS and N₂ adsorption–desorption techniques. The study shows that the rod-like structure is self-assembled CeO₂, and both hydrothermal time and Ce/Cu molar ratio are important factors when the particle-like CeO₂ is being self-assembled into the rod-like CeO₂. The CuO is key active component in the CO-PROX reaction, and its reduction has a negative influence on the selective oxidation of CO. The advantage of the inverse CeO₂/CuO catalyst is that it still can provide sufficient CuO for CO oxidation before 200 °C in the hydrogen-rich reductive gasses. The traditional CuO/CeO₂ catalyst shows better activity at lower temperature and the inverse CeO₂/CuO catalysts present higher CO₂ selectivity when the CO conversion reaches 100%. The performance of mixed sample verifies that they might be complementary in the CO-PROX system.     

CeO2 nanoparticles supported on CuO with petal-like and sphere-flower morphologies for preferential CO oxidation

The CuO supports with different morphology were prepared using the precipitation method. The inverse CeO₂/CuO catalysts were synthesized by the impregnation method, and characterized via XRD, H₂-TPR, SEM, TEM, XPS and N₂ adsorption-desorption techniques. The study showed that CO oxidation took place at the interface of CeO₂–CuO catalysts. The CeO₂/CuO catalysts maintained their morphologies, structure and the length of periphery at the CeO₂–CuO interface in the hydrogen-rich reaction gasses during the reaction. The two-dimensional and homogeneous petal morphology of support was most favorable for the formation of long periphery at the CeO₂–CuO interface, therefore the CeO₂ supported on the CuO with petal morphology presented good catalytic activity.     

Influence of crystallite size and interface on the catalytic performance over the CeO2/CuO catalysts

The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology in order to obtain the CeO₂/CuO catalysts with high BET surface area. The catalysts were characterized via SEM, XRD, H₂-TPR, ICP, HRTEM and N₂ adsorption–desorption techniques. The study shows that CeO₂ and CuO interact on the contact interface. The interaction of oxides switches on CO oxidation at 55 °C and the synergistic effect of interaction also improves H₂ oxidation at 95 °C. CO oxidation takes place at the contact interface of CeO₂ and CuO. The high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO₂ and CuO than the larger CeO₂ particles when most of CeO₂ particles pile into the small clusters and distribute on the bulk CuO. The CeO₂/CuO catalyst with 1:2 Ce/Cu molar ratio has the highest BET surface area and better dispersion of CeO₂ among the catalysts, therefore it display good catalytic activity, selectivity and stability.     

Oxidative steam reforming of methanol over CuO/ZnO/CeO2/ZrO2/Al2O3 catalysts

The composition (CuO/ZnO/Al₂O₃ = 30/60/10) of a commercial catalyst G66B was used as a reference for designing CuO/ZnO/CeO₂/ZrO₂/Al₂O₃ catalysts for the oxidative (or combined) steam reforming of methanol (OSRM). The effects of Al₂O₃, CeO₂ and ZrO₂ on the OSRM reaction were clearly identified. CeO₂, ZrO₂ and Al₂O₃ all promoted the dispersions of CuO and ZnO in CuO/ZnO/CeO₂/ZrO₂/Al₂O₃ catalysts. Aluminum oxide lowered the reducibility of the catalyst, and weakened the OSRM reaction. Cerium oxide increased the reducibility of the catalyst, but weakened the reaction. Zirconium oxide improved the reducibility of the catalyst, and promoted the reaction. A lower CuO/ZnO ratio of the catalyst was associated with greater promotion of ZrO₂. The critical CuO/ZnO ratio for the promotion of ZrO₂ was approximately 0.75–0.8. Introducing of ZrO₂ into CuO/ZnO/Al₂O₃ also improved the stability of the catalyst. Although Al₂O₃ inhibited the OSRM reaction, a certain amount of it was required to ensure the stability and the mechanical strength of the catalysts.     

Water-gas shift reaction over magnesia-modified Pt/CeO2 catalysts

Fuel cells have risen as a clean technology for power generation and much effort has been done for converting renewable feedstock in hydrogen. The water-gas shift reaction (WGS) can be applied aiming at reducing the CO concentration in the reformate. As Pt/CeO₂ catalysts have been pointed out as an alternative to the industrial WGS catalysts, the modification of such systems with magnesium was investigated in this work. It was shown that the addition of MgO to Pt/CeO₂ increased the activity and stability of the catalyst irrespective of the preparation method used, either impregnation or co-precipitation. Based on TPR and IR spectroscopy experiments, it was seen that the presence of magnesium improved ceria reduction favoring the creation of OH groups, which are considered the active sites for the WGS reaction. The evolution of the surface species formed under reaction conditions (CO, H₂O, H₂) observed by DRIFTS evidenced that the formation of formate species and the generation of CO₂ is closely attached to each other; under a reaction stream containing hydrogen the presence of formate species showed to be more relevant while the CO₂ formation was hindered. It is suggested that the addition of MgO favors the formate decomposition and lower the carbonate concentration on the catalyst surface during WGS reaction.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

Comparative study of CuO supported on CeO2, Ce0.8Zr0.2O2 and Ce0.8Al0.2O2 based catalysts in the CO-PROX reaction

CuO supported on CeO_2, Ce_0.8Zr_0.2O₂ and Ce_0.8Al_0.2O₂ based catalysts (6%wt Cu) were synthesized and tested in the preferential oxidation of CO in a H₂-rich stream (CO-PROX). Nanocrystalline supports, CeO₂ and solid solutions of modified CeO₂ with zirconium and aluminum were prepared by a freeze-drying method. CuO was supported by incipient wetness impregnation and calcination at 400 °C. All catalysts exhibit high activity in the CO-PROX reaction and selectivity to CO₂ at low reaction temperature, being the catalyst supported on CeO₂ the more active and stable. The influence of the presence of CO₂ and H₂O was also studied.     

Rare earth oxide modified CuO/CeO2 catalysts for the water–gas shift reaction

Water–gas shift reaction was carried out over a series of CuO/CeO₂ catalysts doped with trivalent rare earth oxide (RE₂O₃, RE = Y, La, Nd and Sm), prepared via co-precipitation method. The effect of the dopants on the structure and catalytic properties of CuO/CeO₂ catalysts was investigated with the aid of X-ray diffraction (XRD), Raman spectra, N₂ physisorption, H₂-TPR and selective N₂O chemisorption characterizations. The results reveal the beneficial role of La₂O₃ and Nd₂O₃ doping in increasing the WGS catalytic activities and stabilities of CuO/CeO₂ catalysts, while the addition of Y₂O₃ and Sm₂O₃ leads to the negative effect. Correlating to the characteristic results, it is found that the performance of CuO/CeO₂–RE₂O₃ catalysts strongly depends on their surface copper dispersion, microstrain value and the amount of oxygen vacancies generated in ceria lattice. Besides, enough evidences suggest that, the most effective active site for WGS reaction is the moderate copper oxide (crystalline) interacted with surface oxygen vacancies of ceria in the CuO–CeO₂ system.     

Water-gas shift reaction over CuO/CeO2 catalysts: Effect of CeO2 supports previously prepared by precipitation with different precipitants

A series of CeO₂ supports were firstly prepared by precipitation method with NH₃⋅H₂O (NH), (NH₄)₂CO₃ (NC) and K₂CO₃ (KC) as precipitant, respectively, and then CuO/CeO₂ catalysts were fabricated by depositing CuO on the as-obtained CeO₂ supports by deposition-precipitation method. The effect of CeO₂ supports prepared from different precipitants on the catalytic performance, physical and chemical properties of CuO/CeO₂ catalysts was investigated with the aid of XRD, N₂-physisorption, N₂O chemisorption, FT-IR, TG, H₂-TPR, CO₂-TPD and cyclic voltammetry (CV) characterizations. The CuO/CeO₂ catalysts were examined with respect to their catalytic performance for the water-gas shift reaction, and their catalytic activities and stabilities are ranked as: CuO/CeO₂-NH > CuO/CeO₂-NC > CuO/CeO₂-KC. Correlating to the characteristic results, it is found that the CeO₂ support prepared by precipitation with NH₃⋅H₂O as precipitant (i.e., CeO₂-NH-300) has the best thermal stability and least surface “carbonate-like” species, which make the corresponding CuO/CeO₂-NH catalyst presents the highest Cu-dispersion, the highest microstrain (i.e., the highest surface energy) of CuO, the strongest reducibility and the weakest basicity. While, the precipitants that contain CO₃^2- (e.g. (NH₄)₂CO₃ and K₂CO₃) result in more surface “carbonate-like” species of CeO₂ supports and CuO/CeO₂ catalysts. As a result, CuO/CeO₂-NC and CuO/CeO₂-KC catalysts present poor catalytic performance.     

Hydrogen production by oxidative steam reforming of methanol over Ni/CeO2–ZrO2 catalysts

Single ZrO₂ and mixed CeO₂-ZrO₂ oxides with different CeO₂/ZrO₂ ratios were prepared by the sol-gel method and the CeO₂ by precipitation. The prepared support were impregnated with an aqueous solution of NiCl₂·6H₂O at an appropriate concentration to yield 3 wt.% of nickel respectively in the catalysts. Catalytic materials were characterized by BET (N₂ adsorption-desorption), SEM-EDS, XRD and TPR. The oxidative steam reforming of methanol (OSRM) reaction was investigated on these catalysts for H₂ production as a function of temperature. Depending of the CeO₂/ZrO₂ ratio; the catalysts composition has a significant influence on the surface area (BET), reduction properties and methanol conversion. XRD patterns of the Ni-base catalysts showed well defined diffraction peaks of the metallic Ni except on the Ni/CeO₂ catalyst, suggesting that on this sample all of the active phase was highly dispersed. Ni/Ceria-rich catalysts were vastly active for OSRM, giving a total CH₃OH conversion at 325 °C with GHSV = 0.3 × 10⁵ h⁻¹. They also showed close selectivity toward H₂, with high selectivity to CO₂ in all range of temperatures, this suggests that the reverse WGS reaction does not occur on these samples. It seems that the nickel is the phase mainly responsible of hydrogen production although the CeO₂/ZrO₂ support reduces the CO formation.     

Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures

CO₂ methanation was performed over 10 wt%Ni/CeO₂, 10 wt%Ni/α-Al₂O₃, 10 wt%Ni/TiO₂, and 10 wt%Ni/MgO, and the effect of support materials on CO₂ conversion and CH₄ selectivity was examined. Catalysts were prepared by a wet impregnation method, and characterized by BET, XRD, H₂-TPR and CO₂-TPD. Ni/CeO₂ showed high CO₂ conversion especially at low temperatures compared to Ni/α-Al₂O₃, and the selectivity to CH₄ was very close to 1. The surface coverage by CO₂-derived species on CeO₂ surface and the partial reduction of CeO₂ surface could result in the high CO₂ conversion over Ni/CeO₂. In addition, superior CO methanation activity over Ni/CeO₂ led to the high CH₄ selectivity.     

Catalytic processes during preferential oxidation of CO in H2-rich streams over catalysts based on copper–ceria

Nanostructured catalysts based on combinations between oxidised copper and cerium entities prepared by two different methods (impregnation of ceria and coprecipitation of the two components within reverse microemulsions) have been examined with respect to their catalytic performance for preferential oxidation of CO in a H₂-rich stream (CO-PROX). Correlations between their catalytic and redox properties are established on the basis of parallel analyses of temperature programmed reduction results employing both H₂ and CO as reactants as well as by XPS. Although general catalytic trends can be directly correlated with the redox properties observed upon separate interactions with each of the two reductants (CO and H₂), the existence of interferences between both reductants must be considered to complete details for such activity/redox correlation. Differences in the nature of the active oxidised copper–cerium contacts present in each case determine the catalytic properties of these systems for the CO-PROX process.     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

H2 production from stable ethanol steam reforming over catalyst of NiO based on flowerlike CeO2 microspheres

Catalyst of nickle oxide based on flowerlike cerium microspheres with high dispersion was made to achieve simultaneous dehydrogenation of ethanol and water molecules on multi-active sites. XRD, 77 K N₂ adsorption and FESEM were applied to analyse and observe the catalyst's structure, porosity and morphology. This special morphology catalyst represented novel stability more than 600 h for hydrogen production at low temperature ethanol steam reforming. Ethanol-water mixtures could be converted into H₂ with average selectivity value of 61.5 mol.% and average ethanol conversion 95.0 mol.% at 350 °C, with GHSV∼1.0 × 10⁵ h⁻¹, low CO selectivity about average value of 2.1 mol.%, during 550 h reforming stability test. Catalytic parameters with respect to yield of H₂, activity, selectivity towards hydrogen production and stability with time on stream were determined.     

Activity,stability and deactivation behavior of Au/CeO2 catalysts in the water gas shift reaction at increased reaction temperature (300 °C)

The effect of increasing the reaction temperature to 300 °C on the activity, stability and deactivation behavior of a 4.5 wt.% Au/CeO₂ catalyst in the water gas shift (WGS) reaction in idealized reformate was studied by kinetic and spectroscopic measurements at 300 °C and comparison with previously reported data for reaction at 180 °C under similar reaction conditions [A. Karpenko, Y. Denkwitz, V. Plzak, J. Cai, R. Leppelt, B. Schumacher, R.J. Behm, Catal. Lett. 116 (2007) 105]. Different procedures for catalyst pretreatment were used, including annealing at 400 °C in oxidative, reductive or inert atmospheres as well as redox processing. The formation/removal of stable adsorbed reaction intermediates and side products (surface carbonates, formates, OH_ad, CO_ad) was followed by in situ IR spectroscopy (DRIFTS), the presence of differently oxidized surface species (Au⁰, Au^0′, Au³⁺, Ce³⁺) was evaluated by XPS. The reaction characteristics at 300 °C generally resemble those at 180 °C, including (i) significantly higher reaction rates, (ii) comparable apparent activation energies (44 ± 1/50 ± 1 kJ mol⁻¹ vs. 40 ± 1 kJ mol⁻¹ at 180 °C), (iii) a correlation between deactivation of the catalyst and the build-up of stable surface carbonates, and (iv) a decrease of the initially significant differences in activity after different pretreatment procedures with reaction time. Different than expected, the tendency for deactivation did not decrease with higher temperature, due to enhanced carbonate decomposition, but increases.     

Photocatalytic hydrogen production over CuO and TiO2 nanoparticles mixture

Cheap and efficient photocatalysts were fabricated by simply mixing TiO₂ nanoparticles (NPs) and CuO NPs. The two NPs combined with each other to form TiO₂/CuO mixture in an aqueous solution due to the opposite surface charge. The TiO₂/CuO mixture exhibited photocatalytic hydrogen production rate of up to 8.23 mmol h⁻¹ g⁻¹ under Xe lamp irradiation when the weight ratio of P25 to CuO was optimized to 10. Although the conduction band edge position of CuO NPs is more positive than normal hydrogen electrode, the TiO₂/CuO mixture exhibited good photocatalytic hydrogen production performance because of the inter-particle charge transfer between the two NPs. The detailed mechanism of the photocatalytic hydrogen production is discussed. This mixing method does not require a complicated chemical process and allows mass production of the photocatalysts.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Hydrogen production from ethanol steam reforming over Ir/CeO2 catalysts: Enhanced stability by PrOx promotion

We studied ethanol steam reforming over Ir/Ce_0.9Pr_0.1O₂ and Ir/CeO₂ catalysts comparatively with respect to activity and stability. We found that PrO_x-doping have significantly promoted the oxygen storage capacity and thermal stability of the catalysts by incorporation into the ceria lattice. Ethanol was readily converted to hydrogen, methane and carbon oxides at 773 K over the Ir/Ce_0.9Pr_0.1O₂ catalyst, and this is 100 K lower than that found for the Ir/CeO₂ catalyst. Moreover, the PrO_x-doped catalyst was stable toward ethanol steam reforming at 923 K for 300 h without an apparent variation in ethanol conversion and product distribution. However, the severe aggregation of ceria particles and heavy coke deposition were observed on the Ir/CeO₂ catalyst, resulting in remarkable deactivation under the same reaction conditions.