A series of ceria oxides doped with 10 mol.% of Zr, Zn and Fe has been prepared by a pseudo sol-gel method throughout the thermal decomposition of the corresponding metallic propionates. With these supports, 1 wt.% gold catalysts were prepared by the deposition-precipitation method. All the solids were characterized by means of XRF, N 2 adsorption, XRD, Raman spectroscopy and SEM techniques and their catalytic activity towards preferential oxidation of CO (PROX) reaction tested. The results showed solid solution when doping with Zr and Fe and ZnO surface segregation in the case of Zn. We demonstrate that gold dispersion not only depends on the oxygen vacancy concentration but also on the nature of the doping agent. Finally, the catalytic activity was highly promoted by gold in all cases, being the doped gold catalysts more active than Au/CeO 2 at low temperature.
GRAPHICAL ABSTRACTThe doping CeO 2 with Zr, Zn and Fe was studied. Doping with Zr or Fe resulted in solid solution while doping with Zn resulted in the surface segregation of ZnO.The formation of oxygen vacancies is enhanced with Zr, remains constant with Zn and disappears on doping with Fe.
The reaction mechanism of reverse water-gas shift (RWGS) reaction was investigated using two commercial gold-based catalysts supported on Al2O3 and TiO2. The surface species formed during the reaction and reaction mechanisms were elucidated by transient and steady-state operando DRIFTS studies. It was revealed that RWGS reaction over Au/Al2O3 proceeds through the formation of formate intermediates that are reduced to CO. In the case of Au/TiO2 catalyst, the reaction goes through a redox mechanism with the suggested formation of hydroxycarbonyl intermediates, which further decompose to CO and water. The Ti 3+ species, the surface hydroxyls, 2 and oxygen vacancies jointly participate. The absence of carbonyl species adsorbed on gold particles during the reaction for both catalysts indicates that the reaction pathway involving dissociative adsorption of CO2 on Au particles can be discarded. To complete the study Operando UV-Vis spectroscopy was successfully applied to confirm the presence of Ti 3+ and to understand the role of the oxygen vacancies of TiO2 support in activating CO2 and thus the subsequent RWGS reaction.
A series of Ce-Fe mixed oxides as well as the pure oxides were synthesized by the microemulsions method. The solid solution formation was established for all the Fedoped systems and only a hardly noticeable segregation of α-Fe 2 O 3 was appreciated for the solid with the maximum iron content (50 Fe atomic%). The oxygen exchange is improved for all the Fe-doped systems; however the 10 Fe atomic% appears as the optimal iron content for achieving the maximum oxygen vacancies concentration and the higher reducibility efficiency. The CO oxidation (TOX, PROX) is especially achieved for the solids with the lower iron contents but with a superior oxygen vacancies proportion. These Ce-Fe systems prepared from microemulsions are very attractive to be considered as supports for depositing active phases capable of enhancing oxygen exchange ability of the whole system, allowing higher CO oxidation abilities.
A series of Ce1−x
O2−x/2 mixed oxides was synthesized by coprecipitation. The solids were characterized by means of XRF, SBET, XRD, UV−vis, and Raman techniques, and their catalytic activities toward CO oxidation were tested. A solid solution, with CeO2 F-type structure, is formed for europium contents (measured as Eu2O3 by XRF) ≤20% wt. For higher contents, the solid solution is not formed, but a physical mixture is detected. The existence of oxygen vacancies in the solids with Eu2O3 contents between 3 and 17% wt was demonstrated by the presence of bands at 532 and 1275 cm−1 in their Raman spectra. The catalytic performances of the solids correlate with the amount of these punctual defects in the solid solution.
Carbon formation and sintering remain the main culprits regarding catalyst deactivation in the dry and bi-reforming of methane reactions (DRM and BRM, respectively). Nickel based catalysts (10 wt.%) supported on alumina (Al2O3) have shown no exception in this study, but can be improved by the addition of tin and ceria. The effect of two different Sn loadings on this base have been examined for the DRM reaction over 20 h, before selecting the most appropriate Sn/Ni ratio and promoting the alumina base with 20 wt.% of CeO2. This catalyst then underwent activity measurements over a range of temperatures and space velocities, before undergoing experimentation in BRM. It not only showed good levels of conversions for DRM, but exhibited stable conversions towards BRM, reaching an equilibrium H2/CO product ratio in the process. In fact, this work reveals how multicomponent Ni catalysts can be effectively utilised to produce flexible syngas streams from CO2/CH4 mixtures as an efficient route for CO2 utilisation
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