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. 2 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.

A series of Ce1−x Eu 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.

The catalytic activity of gold-based catalysts for CO oxidation is influenced by gold particle size, dispersion, and redox properties of the support. The nature of the active site (gold oxidation state) and its modification along the course of the reaction are under discussion. In this work, we studied the modifications of a Au/CeO2 catalyst in the presence of gas-phase CO by simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFT-MS) in isothermal conditions. Redox processes involving surface hydroxyl groups, gold atoms, and gas-phase CO molecules play a determinant role in surface gold dynamics. The interaction of CO with the catalyst surface results in the evolution of CO2 and H2 through both the decomposition of the formate species and the formation of the [Au(CO)2]+ species, which accounts for the redispersion of gold atoms. The identification of the involved surface species by DRIFT lets us state that a deep reduction of the surface (oxygen vacancy creation) changes the gold dispersion and migration of oxygen atoms to the surface, generating an oxidized gold species. This result is also confirmed by XRD, where a decrease in the intensity of all metallic gold diffraction peaks and the relative intensity among them is evidenced in the used catalyst. Therefore, this work provides evidence for the surface dynamics of gold in this Au/CeO2 catalyst and hence provides clues for understanding the modification of the catalytic activity of gold catalysts under cyclic operations.

Iron-modified ceria supports containing different molar percentages of Fe (0%, 10%, 25%, and 50%) were synthesized by thermal decomposition of the metal propionates.

No abstract

A series of Zn-modified ceria solids were prepared by thermal decomposition of the corresponding metal propionates. The formation of segregated ZnO particles on the ceria surface is evidenced for these solids using X-ray diffraction; in addition to this the characterization data may allow discarding the formation of a ZnO-CeO 2 solid solution. On modifying with Zn, the reducibility of the ceria support is enhanced, being the highest reducibility the one obtained for the ZnO-CeO 2 solid having a 1:9 Zn:Ce atomic ratio (CeZn10). The activity of this solid in the CO oxidation reaction was the highest among the tested Zn-modified ceria solids. Therefore, catalysts containing 1 wt.% gold, supported on pure ceria and CeZn solids, were prepared, characterized and their catalytic activities tested.The Zn-modified gold catalyst is more active than the un-modified Au/CeO 2 catalyst in the oxidation of CO; this behavior is related to the higher metallic dispersion of gold on the CeZn support surface. However, the number of oxygen vacancies acting as nucleation sites for gold, is hardly modified in the Zn-modified ceria support and, therefore, the higher gold dispersion must be related to high electron density sites on the catalyst surface as a result of Au-Ce-Zn interaction, this improved gold dispersion results in higher activities for CO oxidation.

Gold-supported ceria and europium-doped ceria catalysts were prepared by the deposition−precipitation method. The influence of the pretreatment atmosphere and temperature on the concentration of oxygen vacancies and gold dispersion on the Au/CeEu(10) catalyst under actual reaction conditions was investigated by “in situ” X-ray diffraction and Raman analysis. By Raman spectroscopy, a preferential interaction of the gold with the support across the oxygen vacancies was established and correlated with the gold dispersion. The increase in the concentration of oxygen vacancies in the presence of hydrogen induces changes in the gold crystallite size by breaking-off and migration of gold nanoparticles toward the oxygen vacancies on the CeEu(10) support. The activity of the Au/CeEu(10) solid in the CO oxidation reaction was significantly improved when the catalyst was preactivated in a reducing atmosphere. This trend could be associated with the redispersion of gold together with the increase in the concentration of oxygen vacancies in the support.

The effect of the magnesia loading on the surface structure and catalytic properties of NiSn/MgO-Al2O3 catalysts for hydrogen production by methanol steam reforming has been investigated. The catalysts have been obtained by impregnation of γ-Al2O3 by the incipient wetness method, with variation of the MgO content. X-ray diffraction (XRD), BET surface area and H2-temperature programmed reduction (TPR) have been used to characterise the prepared catalysts. From this, it has been concluded that the incorporation of MgO results in the formation of MgAl2O4 spinel, which modifies the acid-base properties of the catalysts. The formation of Ni-Sn alloys after the reductive pre-treatment has also been evidenced.The influence of the temperature of reaction and of the MgO loading on the hydrogen production by reforming of methanol has been established. Moreover, tests of catalytic stability have been carried out for more than 20 h. The carbonaceous deposits have been examined by temperature-programmed oxidation (TPO). The analysis of the catalysts after reaction has confirmed the low level of carbon formation on these catalysts. In no case, carbon nanotubes have been detected on the solids.