Display omitted
•Band gap engineering of CeO2-x nanoparticles shifts absorption from UV to visible.•CeO2-x nanoparticles of high surface area and surface O vacancy population used for H2 ...production.•Improving the activity of CeO2-x nanoparticles for the photocatalytic H2 production.
High surface area cerium oxide (CeO2-x) nanoparticles (S = 170 m2/g) were synthesized by the precipitation method with a narrow band gap (2.73 ± 0.03 eV). In comparison to typical band gap values for cerium oxide, it presents a red shift in the light absorption spectrum from UV to visible region. X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Absorption Spectroscopy (XAS), X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) measurements were conducted aiming to elucidate this promising property for photocatalytic applications of the nanoparticles synthesized. It was obtained that the high structural disorder and O vacancy population of the nanoparticles synthesized are responsible for the narrow band gap found. Furthermore, the CeO2-x nanoparticles were applied to the photocatalytic H2 production reaction and presented activity 10 times higher than the commercial CeO2-x standard, besides a much better performance than typical results found for CeO2-x in the literature.
Display omitted
•Rational design of catalyst by means of a CO reduction treatment.•Very high Ce(III) fraction values obtained by CO reduction treatment.•Surface hydrophobicity plays an important role ...in the CeO2-x nanoparticles reduction.•Low reduction temperatures values found for CeO2-x nanoparticles.
Oxygen vacancies are one of the most active defects existing at the surface of metal oxides. Engineering of defect chemistry, especially oxygen vacancies, represents the possibility of controlling the catalytic properties of nanoparticles. This work was dedicated to study the reduction properties of CeO2-x nanoparticles prepared using different synthesis parameters when exposed to a CO reducing atmosphere. Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) and in situ time-resolved X-Ray Absorption Near Edge Spectroscopy (XANES) techniques were used to probe in situ the electronic properties of these nanoparticles exposed to the CO atmosphere. Compared to typical results found for CeO2-x nanoparticles, an improved reduction tendency is observed for these synthesized nanoparticles. It was possible to tune the oxygen vacancy population and the reduction temperature (TR) by means of the combination of synthesis and CO reducing atmosphere. The TR and Ce(III) fraction values reached can be as low as 108°C and as high as 0.92, respectively. The influence of structural and electronic properties of the nanoparticles on the reduction kinetics of the CeO2-x reduction is elucidated. The results provide a valuable route for the rational design of CeO2-x nanoparticles with the desired oxygen vacancy population, and then preparing them in the best conditions for applications.
Ceria (CeO2) is being increasingly used as support of metallic nanoparticles in catalysis due to its unique redox properties. Shedding light into the nature of the strong metal support interaction ...(SMSI) effect in CeO2-containing catalysts is important since it has a strong influence on the catalytic properties of the system. In this work, Cu/CeO2 and Ni/CeO2 nanoparticles are characterized when submitted to a reduction treatment at 500 °C in H2 atmosphere with a combination of in situ (XAS – X-ray absorption spectroscopy and time-resolved XAS) and ex situ (TEM – transmission electron microscopy and XPS - X-ray photoelectron spectroscopy) techniques. The existence of a capping layer decorating the Ni/CeO2 nanoparticles after the reduction treatment is shown, representing evidence for the SMSI effect. The kinetics of the SMSI occurrence is elucidated. It is proposed that the electronic factor of the SMSI effect has a strong influence on the reduction properties of the Ni nanoparticles supported on CeO2, decreasing its reduction temperature if compared to nonsupported Ni nanoparticles. The same phenomenon is not observed for Cu/CeO2 nanoparticles, where there is no evidence for the SMSI effect, and no changes on the reduction properties between supported and nonsupported Cu nanoparticles are observed.
We have been able to “tune” the electrocatalytic activity of iron phthalocyanine (FePc) and iron hexadodecachlorophthalocyanine (16(Cl)FePc) for the oxygen reduction reaction (ORR) by manipulating ...the “pull effect” of pyridinium molecules axially bounded to the phthalocyanine complexes (FePcs). These axial ligands play both the role of molecular anchors and also of molecular wires. The axial ligands also affect the reactivity of the Fe metal center in the phthalocyanine. The “pull effect” originates from the positive charge located on the pyridinium core. We have explored the influence of the core positions (Up or Down), in two structural pyridiniums isomers on the activity of FePc and 16(Cl)FePc for the ORR. Of all self-assembled catalysts tested, the highest catalytic activity was exhibited by the Au(111)/Up/FePc system. XPS measurements and DFT calculations showed that it is possible to tailor the FePc–N(pyridiniums) Fe–O2 binding energies, by changing the core positions and affecting the “pull effect” of pyridiniums. This affects directly the catalytic activity of FePcs. The plot of activity as (log I)E versus the calculated Fe–O2 binding energies gives an activity volcano correlation, indicating that an optimum binding energy of O2 with the Fe center provides the highest activity.
Here, we report the Pt nanoparticle mediated reduction (oxidation) and lattice expansion (contraction) of mesoporous CeO2 under H2 (O2) atmospheres and in the temperature range of 50–350 °C. We found ...that CeO2 in the Pt/CeO2 catalyst was partially reduced in H2 (and fully oxidized back in O2) as demonstrated by several in situ techniques: APXPS spectra (4d core levels) for the topmost surface, NEXAFS total electron yield spectra (at the M5,4 edges) in the near surface regions, and (N)EXAFS fluorescence spectra (at the L3 edge) in the bulk. Moreover, XRD and EXAFS showed the reversible expansion and contraction of the CeO2 unit cell in H2 and O2 environments, respectively. The expansion of the CeO2 cell was mainly associated with the formation of oxygen vacancies as a result of the Pt-mediated reduction of Ce4+ to Ce3+. We also found that pure mesoporous CeO2 can not be reduced in H2 under identical conditions but can be partially reduced at above 450 °C as revealed by APXPS. The role of Pt in H2 was identified as a catalytic one that reduces the activation barrier for the reduction of CeO2 via hydrogen spillover.
A detailed investigation concerning the atomic structure of Cr2O3 and Pd/Cr2O3 ultrathin films deposited on a Ag(111) single crystal is presented. The films were prepared by MBE (molecular beam ...epitaxy) and characterized in situ by LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy), and XPD (X-ray photoelectron diffraction). Evidences of rotated domains and an oxygen-terminated Cr2O3/Ag(111) surface were observed, along with significant contractions of the oxide’s outermost interlayer distances. The deposition of Pd atoms on the Cr2O3 surface formed a four-monolayer film, fcc packed and oriented in the 111 direction, which presented changes in monolayer spacing and lateral atomic distance compared to the expected values for bulk Pd. The observed surface structure may shed light on new physical properties such as the induced magnetic ordering in Pd atoms.
In the present work, we report the obtaining of nanostructured porous Nb2O5 by anodizing process, with Pt, Ta, Cu, or Ti impregnation, and by magnetron sputtering process. The techniques of scanning ...electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy were used to evaluate the morphology, and the composition and crystalline structure of nanoporous Nb2O5. The photocatalytic activity was evaluated by electrochemical photocurrent tests under UV light illumination. Nanostructured porous Nb2O5 with Pt, Ta, Cu, and Ti impregnation presented bandgap reduction and 10 times higher photocurrent density in the presence of UV light when compared to the not impregnated nanoporous Nb2O5.
Graphical :
•Nanometric Si-based films were obtained by argon plasma polymerization.•Nanometer film was formed predominantly by Si-O and organosilane bonds.•Films deposited on the nanostructures showed greater ...roughness and wettability.•Sanded and plasma polymerized samples showed a higher number of viable stem cells.
The modification of surfaces by the application of thin films has been used in the regenerative medicine area to increase the biocompatibility of metal implants. Titanium alloy has been recently used as substrate in polymerization for biomedical application. In this context, silane films were obtained by plasma polymerization in favor of the sol-gel method and the influence of different pressures in obtaining these films by argon plasma polymerization was evaluated from the alkoxysilane precursor tetraethoxysilane (TEOS) on the Ti6Al4V alloy. The morphological characterization of the films was performed by AFM, Profilometry and Spectral Ellipsometry and the chemical composition was analyzed by XPS. The biological behavior was evaluated by analyzing the mitochondrial activity and cellular viability of mesenchymal stem cells. The plasma polymerization process resulted in the deposition of a nanometric Si-based film formed, predominantly, by Si-O and organosilane bonds. The films that were applied on a sanded surface, with lower pressures in the plasma polymerization process, presented a lower layer thickness and wettability than the films obtained on nanotextured surfaces. Considering absorbance values, the Ti6Al4V samples mechanically sanded and deposited by plasma polymerization at 230 µatm presented better cell viability than samples with nanotextured surfaces coated with plasma polymerized film, indicating this material has potential to biomedical application.
Display omitted
In this work, the reduction properties of ceria (CeO2) used as support of metallic nanoparticles (Au, Pd, Au0.9Pd0.1, and Au0.8Pt0.2) were elucidated. The catalysts were exposed to a reduction ...treatment in H2 atmosphere at 500 °C. In situ X-ray absorption spectroscopy (in situ XAS) and in situ time-resolved XAS measurements at the Ce L3 edge were used to probe the local atomic order around Ce atoms and the Ce oxidation state. In this way, it was observed that the supported metallic nanoparticles improve the reduction process of the support. Moreover, the reduction of ceria is dependent on the composition of the metallic nanoparticle supported. By means of in situ XAS measurements at the Au and Pt L2,3 edges, it was possible to obtain information concerning the fractional change in the number of 5d-band electron holes relative to a reference material for the Au- and Pt-containing nanoparticles. In this way, it was observed as evidence for the charge transfer effect from the nanoparticles to the support, which is responsible for the improved reduction of the CeO2 support in the presence of nanoparticles. This result is corroborated by the observation of energy shifts on the Au 4f, Pd 3d, and Pt 4f binding energy values of the measured X-ray photoelectron spectroscopy (XPS) spectra. In this way, this work contributes to elucidating the physical mechanisms responsible for the enhanced support reduction effect existing in modern ceria-based catalyst.
PDF