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•Bimetallic PdNb catalysts show promising results in the dry reforming of methane.•A synergic effect between Pd and Nb is observed.•The addition of Nb improved the catalytic activity ...minimizing carbon deposits formation.•The catalytic stability depended on the reoxidation degree of active sites.
This work describes the use of Pd-Nb catalysts supported on commercial silica in the dry reforming of methane. The effect of Pd/Nb ratio was evaluated and it determined the catalytic response of the prepared catalysts. Thus, it was observed that the catalytic activity increased with the Pd/Nb ratio and bimetallic samples were more active than the monometallic ones. The sample with the highest Pd/Nb ratio was the best in terms of conversion, selectivity and stability. Nb presence decreased the amount of carbon deposited on the catalysts since the decomposition of CH4 is minimised in the operating conditions.
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•CeO2/YSZ oxide thermo-chemically activated showing very high reducibility.•High activity and stability of Ni/CeO2/YSZ catalyst for the reforming of methane with CO.•Deactivating ...forms or carbon avoided by using highly reducible oxides.
Ni/CeO2/YSZ and Ni/Ce0.15Zr0.85O2 have been investigated as catalysts for the dry reforming of methane at 750°C. Ni was incorporated by the impregnation method. The supports were previously activated by using a thermo-chemical protocol consisting on a severe reduction (H2/Ar) at 950°C followed by a mild oxidation (O2/He) at 500°C. According to TPR results, this protocol leads to the development of unique redox properties in the case of the CeO2/YSZ oxide. Two types of CO2+CH4 (1:1) mixtures (helium-diluted and undiluted) were used to feed the reactor. When using the Ni/Ce0.15Zr0.85O2 catalyst with undiluted feed, the reactor became plugged by coke. By contrast, Ni/CeO2/YSZ behaved as an active and stable catalyst even under the most severe operation conditions. The characterization of the spent Ni/CeO2/YSZ using TGA, TEM, Raman and XPS spectroscopy revealed that only a limited amount of graphitic carbon, in form of nanotubes, was formed. No evidences of deactivating carbonaceous forms were obtained. The singular redox properties of the activated CeO2/YSZ oxides are proposed as a key for designing Ni catalysts highly stable in reforming processes.
This work is devoted to evaluating the relationship between the oxygen content and catalytic activity in the CO oxidation process of the 6H-type BaFeO
system. Strong evidence is provided about the ...improvement of catalytic performance with increasing Fe average oxidation state, thus suggesting the involvement of lattice oxygen in the catalytic process. The compositional and structural changes taking place in both the anionic and cationic sublattices of the catalysts during redox cycles have been determined by temperature-resolved neutron diffraction. The obtained results evidence a structural transition from hexagonal (
6
/
) to orthorhombic (
) symmetry. This transition is linked to octahedra distortion when the Fe
concentration exceeds 40% (δ values higher than 0.2). The topotactical character of the redox process is maintained in the δ range 0 < δ < 0.4. This suggests that the cationic framework is only subjected to slight structural modifications during the oxygen exchange process occurring during the catalytic cycle.
This work is devoted to evaluating the relationship between the oxygen content and catalytic activity in the CO oxidation process of the 6H-type BaFeO3−δ system. Strong evidence is provided about the ...improvement of catalytic performance with increasing Fe average oxidation state, thus suggesting the involvement of lattice oxygen in the catalytic process. The compositional and structural changes taking place in both the anionic and cationic sublattices of the catalysts during redox cycles have been determined by temperature-resolved neutron diffraction. The obtained results evidence a structural transition from hexagonal (P63/mmc) to orthorhombic (Cmcm) symmetry. This transition is linked to octahedra distortion when the Fe3+ concentration exceeds 40% (δ values higher than 0.2). The topotactical character of the redox process is maintained in the δ range 0 < δ < 0.4. This suggests that the cationic framework is only subjected to slight structural modifications during the oxygen exchange process occurring during the catalytic cycle.
The contribution of electron microscopy techniques to establishing the existence and actual nature of the metal–support interaction effects occurring in a variety of supported metal catalysts is ...reviewed. Data pertaining to systems based on both classic reducible supports like TiO
2, CeO
2 or some ceria-based mixed oxides, and several others generally considered as non-reducible oxides, like La
2O
3 or SiO
2, are presented and discussed. The specific temperature and chemical conditions under which strong metal–support interaction phenomena are onset or reverted in each case are also analysed.
The whole set of data presented and discussed here clearly shows that the electron microscopy techniques have made an outstanding contribution to the characterisation of the strong metal/support interaction effects exhibited by different metal/oxide systems. Likewise, it is demonstrated that this powerful family of techniques has very much helped to discriminating between
true SMSI-like phenomena, as originally defined by Tauster et al., and several other
apparent effects which, though at a first sight show some of the chemical, nano-structural and/or catalytic characteristics of the SMSI effect, have a neatly different origin.
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•Different structure and reducibility in Ce/Zr oxides due to different treatment.•Study of surface properties of Ce/Zr mixed oxides with different reducibility.•Surface ...characterization carried out by XPS and adsorption of probe molecules.•Strong effect on redox behaviour due to slight differences in surface.•FLNS using Eu3+ dopants proved to be a sensitive structural tool.
This paper presents a study of the surface properties of two Ce/Zr mixed oxides with different reducibility, obtained by applying distinct thermal ageing treatments to an oxide with the composition Ce0.62Zr0.38O2. The surface composition was investigated by XPS. Chemical reactivity of the surface was studied by adsorption of the probe molecules CO2, D2 and methanol. Nanostructural characterization was carried out by XRD, Raman and high-resolution Eu3+ spectroscopy (FLNS). The characterization showed only slight variations in surface composition and bulk Ce-Zr distribution, but hardy differences concerning the type and strength of acidic surface centres, as well as strong differences in the ability to dissociate hydrogen. Structural variations between both samples were identified by comparing the optical spectra of Eu3+ in surface doped samples.
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•STEM-XEDS enables direct visualization of small copper nanostructures in catalysts.•It is possible to measure dispersion of copper by hydrogen chemisorption at 100 °C.•Catalyst with ...high copper dispersion is the most active at low temperature in CO-PROX.
The preferential oxidation of CO in H2-rich mixtures has been studied using Ce0.8Zr0.2O2-supported Cu catalysts containing increasing amounts of copper 1 wt% (Cu1/CZ), 2 wt% (Cu2/CZ) and 4 wt% (Cu4/CZ). The oxide support was prepared by co-precipitation and the copper was incorporated using the wetness impregnation method. The catalysts were characterized using N2 adsorption, XRD, STEM-XEDS, H2-TPR, H2 chemisorption and XPS techniques. In particular, the complementarity of electron microscopy and hydrogen chemisorption techniques was found to be very useful for elucidating one of the most challenging properties of copper catalysts, namely their dispersion. The results show that the Cu1/CZ catalyst is the most active in the CO-PROX reaction at low temperatures (<125 °C), despite having a lower copper content. The results are discussed and correlated with the textural properties of the catalysts and with parameters such as metal dispersion and the binary interfacial active sites.
A solvothermal route to prepare Glutathione capped hybrid ytterbium/iron oxide nanoparticles with potential applications as multiplatform contrast agent in medical image techniques has been ...developed. The influence of ytterbium/iron molar ratio used as precursor, as well as the degree of the autoclave filling on the structural and morphological characteristics of the obtained nanoparticles has been extensively studied. Although all nanoparticles present similar composition, with YbFeO3 being the majority phase, size and morphology of the as synthetized nanoparticles are highly influenced by the critical temperature and by the over -saturation reached during the solvothermal process. We have demonstrated that glutathione properly functionalizes the hybrid nanoparticles, increasing their colloidal stability and decreasing their cytotoxicity. Additionally, they show good imaging in magnetic resonance and X-ray computerized tomography, thereby indicating promising potential as a dual contrast agent. This work presents, for the first time, glutathione functionalized ytterbium/iron oxide nanoparticles with potential applications in Biomedicine.
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